Method for solid freeform fabrication

ABSTRACT

Methods of layerwise fabrication of a three-dimensional object, and objected obtained thereby are provided. The methods are effected by dispensing at least a first modeling formulation and a second modeling formulation to form a core region using both said first and said second modeling formulations, an inner envelope region at least partially surrounding said core region using said first modeling formulation but not said second modeling formulation, and an outer envelope region at least partially surrounding said inner envelope region using said second modeling formulations but not said first modeling formulation; and exposing said layer to curing energy, thereby fabricating the object, The first and second modeling formulations are selected such they differ from one another, when hardened, by at least one of Heat Deflection Temperature (HDT), Izod Impact resistance, Tg and elastic modulus.

RELATED APPLICATIONS

This application is a National Phase of PCT Patent Application No.PCT/IB2017/055692 having International filing date of Sep. 20, 2017,which claims the benefit of priority under 35 USC § 119(e) of U.S.Provisional Patent Application No. 62/397,952 filed on Sep. 22, 2016.The contents of the above applications are all incorporated by referenceas if fully set forth herein in their entirety.

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates to AdditiveManufacturing (AM) of an object, and, more particularly, but notexclusively, to methods and systems for additive manufacturing of alayered object.

Additive manufacturing is generally a process in which athree-dimensional (3D) object is manufactured utilizing a computer modelof the objects. Such a process is used in various fields, such as designrelated fields for purposes of visualization, demonstration andmechanical prototyping, as well as for rapid manufacturing (RM).

The basic operation of any AM system consists of slicing athree-dimensional computer model into thin cross sections, translatingthe result into two-dimensional position data and feeding the data tocontrol equipment which manufacture a three-dimensional structure in alayerwise manner.

Various AM technologies exist, amongst which are stereolithography,digital light processing (DLP), and three dimensional (3D) printing, 3Dinkjet printing in particular. Such techniques are generally performedby layer by layer deposition and solidification of one or buildingmaterials, typically photopolymerizable (photocurable) materials.

Stereolithography, for example, is an additive manufacturing processwhich employs a liquid UV-curable building material and a UV laser. Insuch a process, for each dispensed layer of the building material, thelaser beam traces a cross-section of the part pattern on the surface ofthe dispensed liquid building material. Exposure to the UV laser lightcures and solidifies the pattern traced on the building material andjoins it to the layer below. After being built, the formed parts areimmersed in a chemical bath in order to be cleaned of excess buildingmaterial and are subsequently cured in an ultraviolet oven.

In three-dimensional inkjet printing processes, for example, a buildingmaterial is dispensed from a dispensing head having a set of nozzles todeposit layers on a supporting structure. Depending on the buildingmaterial, the layers may then be cured or solidified using a suitabledevice.

Various three-dimensional inkjet printing techniques exist and aredisclosed in, e.g., U.S. Pat. Nos. 6,259,962, 6,569,373, 6,658,314,6,850,334, 7,183,335, 7,209,797, 7,225,045, 7,300,619, 7,479,510,7,500,846, 7,962,237, 9,031,680 and U.S. Patent Application havingPublication No. 2015/0210010, all of the same Assignee.

A printing system utilized in additive manufacturing may include areceiving medium and one or more printing heads. The receiving mediumcan be, for example, a fabrication tray that may include a horizontalsurface to carry the material dispensed from the printing head. Theprinting head may be, for example, an ink jet head having a plurality ofdispensing nozzles arranged in an array of one or more rows along thelongitudinal axis of the printing head. The printing head may be locatedsuch that its longitudinal axis is substantially parallel to theindexing direction. The printing system may further include acontroller, such as a microprocessor to control the printing process,including the movement of the printing head according to a pre-definedscanning plan (e.g., a CAD configuration converted to a StereoLithography (STL) format and programmed into the controller). Theprinting head may include a plurality of jetting nozzles. The jettingnozzles dispense material onto the receiving medium to create the layersrepresenting cross sections of a 3D object.

In addition to the printing head, there may be a source of curingenergy, for curing the dispensed building material. The curing energy istypically radiation, for example, UV radiation.

Additionally, the printing system may include a leveling device forleveling and/or establishing the height of each layer after depositionand at least partial solidification, prior to the deposition of asubsequent layer.

The building materials may include modeling materials and supportmaterials, which form the object and the temporary support constructionssupporting the object as it is being built, respectively.

The modeling material (which may include one or more material) isdeposited to produce the desired object/s and the support material(which may include one or more materials) is used, with or withoutmodeling material elements, to provide support structures for specificareas of the object during building and assure adequate verticalplacement of subsequent object layers, e.g., in cases where objectsinclude overhanging features or shapes such as curved geometries,negative angles, voids, and so on.

Both the modeling and support materials are preferably liquid at theworking temperature at which they are dispensed, and subsequentlyhardened, typically upon exposure to curing energy (e.g., UV curing), toform the required layer shape. After printing completion, supportstructures are removed to reveal the final shape of the fabricated 3Dobject.

Several additive manufacturing processes allow additive formation ofobjects using more than one modeling material. For example, U.S. PatentApplication having Publication No. 2010/0191360 of the present Assignee,discloses a system which comprises a solid freeform fabricationapparatus having a plurality of dispensing heads, a building materialsupply apparatus configured to supply a plurality of building materialsto the fabrication apparatus, and a control unit configured forcontrolling the fabrication and supply apparatus. The system has severaloperation modes. In one mode, all dispensing heads operate during asingle building scan cycle of the fabrication apparatus. In anothermode, one or more of the dispensing heads is not operative during asingle building scan cycle or part thereof.

In a 3D inkjet printing process such as Polyjet™ (Stratasys Ltd.,Israel), the building material is selectively jetted from one or moreprinting heads and deposited onto a fabrication tray in consecutivelayers according to a pre-determined configuration as defined by asoftware file.

When a cured rigid modeling material forms the final object, the curedmaterial should preferably exhibit heat deflection temperature (HDT)which is higher than room temperature, in order to assure its usability.Typically, the cured modeling material should exhibit HDT of at least35° C. For an object to be stable in variable conditions, a higher HDTis desirable.

U.S. Patent Application having Publication No. 2013/0040091, by thepresent assignee, discloses methods and systems for solid freeformfabrication of shelled objects, constructed from a plurality of layersand a layered core constituting core regions and a layered shellconstituting envelope regions.

SUMMARY OF THE INVENTION

According to an aspect of some embodiments of the present inventionthere is provided a method of layerwise fabrication of athree-dimensional object, the method comprising, for each of at least afew of the layers:

dispensing at least a first modeling formulation and a second modelingformulation to form a core region using both the first and the secondmodeling formulations, an inner envelope region at least partiallysurrounding the core region using the first modeling formulation but notthe second modeling formulation, and an outer envelope region at leastpartially surrounding the inner envelope region using the secondmodeling formulations but not the first modeling formulation;

exposing the layer to curing energy, thereby fabricating the object,

wherein each of the first modeling formulation and the second modelingformulation comprises at least one UV-curable material, and wherein thefirst modeling formulation and the second modeling formulation differfrom one another, when hardened, by at least one of:

Heat Deflection Temperature (HDT), Izod Impact resistance, Tg andelastic modulus.

According to some of any of the embodiments described herein, an HDT ofthe first modeling material formulation, when hardened, is higher thanan HDT of the second modeling material formulation, when hardened.

According to some of any of the embodiments described herein, an HDT ofthe second modeling material formulation, when hardened, is lower than50° C. and an HDT of the first modeling material formulation, whenhardened, is higher than 50° C.

According to some of any of the embodiments described herein, an IzoDImpact Resistance of the second modeling material formulation, whenhardened, is higher than an Izod Impact Resistance of the first modelingmaterial formulation, when hardened.

According to some of any of the embodiments described herein, a ratiobetween elastic moduli of the first modeling material formulation andthe second modeling formulation, when hardened, ranges from 1 to 20, orfrom 1 to 10, or from 1 to 5, or from 2 to 5, or from 2 to 3, or from2.5 to 3, or from 2.7 to 2.9.

According to some of any of the embodiments described herein, the firstmodeling material formulation comprises at least one curable materialthat is characterized, when hardened, by Tg of at least 50° C., asdescribed herein in any of the respective embodiments.

According to some of any of the embodiments described herein, the firstmodeling material formulation comprises at least two curable materials,at least one of the curable materials is characterized, when hardened,by Tg of at least 80° C., as described herein in any of the respectiveembodiments.

According to some of any of the embodiments described herein, the firstmodeling material formulation comprises at least two curable materials,at least one of the curable materials is characterized, when hardened,by Tg of at least 100° C., or at least 150° C., as described herein inany of the respective embodiments.

According to some of any of the embodiments described herein, the firstmodeling material formulation comprises: at least one curable(meth)acrylic monomer; at least one curable (meth)acrylic oligomer; andoptionally, at least one curable (meth)acrylic monomer characterized,when hardened, by Tg lower than 0° C., as described herein in any of therespective embodiments.

According to some of any of the embodiments described herein, the firstmodeling material formulation comprises: at least one curable(meth)acrylic monomer characterized, when hardened, by Tg of at least85° C.; at least one curable (meth)acrylic monomer characterized, whenhardened, by Tg of at least 150° C.; at least one curable (meth)acrylicoligomer, characterized, when hardened, by Tg of at least 50° C.; andoptionally, at least one curable (meth)acrylic monomer characterized,when hardened, by Tg lower than 0° C., as described herein in any of therespective embodiments.

According to some of any of the embodiments described herein, the secondmodeling material formulation comprises at least two curable materials,at least one of the curable materials is a (meth)acrylic monomercharacterized, when hardened, by Tg lower than −20° C., as describedherein in any of the respective embodiments.

According to some of any of the embodiments described herein, the secondmodeling material formulation further comprises at least one curable(meth)acrylic monomer characterized, when hardened, by Tg of at least70° C., as described herein in any of the respective embodiments.

According to some of any of the embodiments described herein, the secondmodeling material formulation further comprises at least one curable(meth)acrylic oligomer characterized, when hardened, by Tg of at least10° C., as described herein in any of the respective embodiments.

According to some of any of the embodiments described herein, the coreregion comprises a voxelated combination between the first and thesecond modeling formulations.

According to some of any of the embodiments described herein, athickness of the inner envelope region, as measured within a plane ofthe layer and perpendicularly to a surface of the object, is preferablyfrom about 0.1 mm to about 4 mm.

According to some of any of the embodiments described herein, athickness of the outer envelope region, as measured within a plane ofthe layer and perpendicularly to a surface of the object, is from aboutfrom about 150 microns to about 600 microns.

According to some of any of the embodiments described herein, thedispensing is executed to form at least one additional envelope regionbetween the inner envelope region and the outer envelope region.

According to some of any of the embodiments described herein, thedispensing of the additional envelope region is using both the first andthe second modeling formulations.

According to some of any of the embodiments described herein, athickness of the additional envelope, as measured within a plane of thelayer and perpendicularly to a surface of the object, is less than athickness of the inner envelope region and also less than a thickness ofthe outer envelope region.

According to some of any of the embodiments described herein, athickness of the additional envelope, as measured within a plane of thelayer and perpendicularly to a surface of the object, is from about 70microns to about 100 microns.

According to some of any of the embodiments described herein, a ratiobetween a number of voxels within the additional envelope region thatare occupied by the first modeling formulation and a number of voxelswithin the additional envelope region that are occupied by the secondmodeling formulation is about 1.

According to some of any of the embodiments described herein, the methodfurther comprises dispensing a plurality of base layers to form a basesection of the object, the plurality of base layers comprising at leastone outer base layer made of the second modeling formulation but not thefirst modeling formulation, and at least one inner base layer made ofthe first modeling formulation but not the second modeling formulation.

According to some of any of the embodiments described herein, an overallthickness of the at least one outer base layer along a build directionof the object approximately equals to a thickness of the outer enveloperegion as measured in a plane engaged by the outer envelope region andperpendicularly to a surface of the object.

According to some of any of the embodiments described herein, theplurality of base layers comprises at least one intermediate base layerbetween the at least one inner base layer and the at least one outerbase layer, the intermediate base layer being made of both the firstmodeling formulation and the second modeling formulation.

According to some of any of the embodiments described herein, the methodfurther comprises dispensing a plurality of top layers to form a topsection of the object, the plurality of top layers comprising at leastone outer top layer made of the second modeling formulation but not thefirst modeling formulation, and at least one inner top layer made of thefirst modeling formulation but not the second modeling formulation.

According to some of any of the embodiments described herein, an overallthickness of the at least one outer top layer along a build direction ofthe object approximately equals to a thickness of the outer enveloperegion as measured in a plane engaged by the outer envelope region andperpendicularly to a surface of the object.

According to some of any of the embodiments described herein, theplurality of top layers comprises at least one intermediate top layerbetween the at least one inner top layer and the at least one outer toplayer, the intermediate top layer being made of both the first modelingformulation and the second modeling formulation.

According to some of any of the embodiments described herein, at leastone parameter characterizing the first formulation is selected toprovide a predetermined damping for the core.

According to some of any of the embodiments described herein, the atleast one parameter comprises an extent of cross linking of the firstmodeling formulation.

According to some of any of the embodiments described herein, the atleast one parameter comprises a total calculated Tg of the firstformulation, as calculated by summing individual Tg values of polymericmaterials included in the first modeling formulation, when hardened.

According to some of any of the embodiments described herein, relativeamounts of the first and the second formulations is selected to providea predetermined damping for the core.

According to an aspect of some embodiments of the present inventionthere is provided a three-dimensional object obtained in layerwise solidfreeform fabrication, the object comprising a plurality of layers, atleast one layer comprising:

a core region made, at least in part, of a first hardened modelingmaterial formed of a first modeling formulation and a second modelingformulation, an inner envelope region at least partially surrounding thecore region and being made, at least in part, of a second hardenedmodeling material formed of the first modeling formulation but not thesecond modeling formulation, and an outer envelope region at leastpartially surrounding the inner envelope region and being made, at leastin part, of a third hardened modeling material formed of the secondmodeling formulation but not the first modeling formulation;

wherein each of the first modeling formulation and the second modelingformulation comprises at least one UV-curable material, and wherein thefirst modeling formulation and the second modeling formulation differfrom one another, when hardened, by at least one of:

Heat Deformation Temperature (HDT), Izod Impact resistance and elasticmodulus.

According to some of any of the embodiments described herein, the coreregion comprises a voxelated combination between a hardened materialformed of the first modeling material formulation and a hardenedmaterial formed of the second modeling material formulation.

According to some of any of the embodiments described herein, the objectfurther comprises at least one additional envelope region between theinner envelope region and the outer envelope region.

According to some of any of the embodiments described herein, theadditional envelope region is made, at least in part, of a fourthhardened modeling material formed of both the first and the secondmodeling formulations.

According to some of any of the embodiments described herein, a ratiobetween a number of voxels within the additional envelope region thatare occupied by a hardened material formed of the first modelingformulation and a number of voxels within the additional envelope regionthat are occupied by a hardened material formed of the second modelingformulation is about 1.

According to some of any of the embodiments described herein, the objectfurther comprises a plurality of base layers forming a base section ofthe object, the plurality of base layers comprising at least one outerbase layer made of a hardened material formed of the second modelingformulation but not the first modeling formulation, and at least oneinner base layer made of a hardened material formed of the firstmodeling formulation but not the second modeling formulation.

According to some of any of the embodiments described herein, an overallthickness of the at least one outer base layer along a build directionof the object approximately equals to a thickness of the outer enveloperegion as measured in a plane engaged by the outer envelope region andperpendicularly to a surface of the object.

According to some of any of the embodiments described herein, theplurality of base layers comprises at least one intermediate base layerbetween the at least one inner base layer and the at least one outerbase layer, the intermediate base layer being made of a hardenedmaterial formed of both the first modeling formulation and the secondmodeling formulation.

According to some of any of the embodiments described herein, the objectfurther comprises a plurality of top layers forming a top section of theobject, the plurality of top layers comprising at least one outer toplayer made of a hardened modeling material formed of the second modelingformulation but not the first modeling formulation, and at least oneinner top layer made of a hardened modeling material formed of the firstmodeling formulation but not the second modeling formulation.

According to some of any of the embodiments described herein, an overallthickness of the at least one outer top layer along a build direction ofthe object approximately equals to a thickness of the outer enveloperegion as measured in a plane engaged by the outer envelope region andperpendicularly to a surface of the object.

According to some of any of the embodiments described herein, theplurality of top layers comprises at least one intermediate top layerbetween the at least one inner top layer and the at least one outer toplayer, the intermediate top layer being made of hardened material formedof both the first modeling formulation and the second modelingformulation.

According to some of any of the embodiments described herein, the firstmodeling material formulation and the second modeling materialformulations are as described herein in any of the respectiveembodiments and any combination thereof.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

Implementation of the method and/or system of embodiments of theinvention can involve performing or completing selected tasks manually,automatically, or a combination thereof. Moreover, according to actualinstrumentation and equipment of embodiments of the method and/or systemof the invention, several selected tasks could be implemented byhardware, by software or by firmware or by a combination thereof usingan operating system.

For example, hardware for performing selected tasks according toembodiments of the invention could be implemented as a chip or acircuit. As software, selected tasks according to embodiments of theinvention could be implemented as a plurality of software instructionsbeing executed by a computer using any suitable operating system. In anexemplary embodiment of the invention, one or more tasks according toexemplary embodiments of method and/or system as described herein areperformed by a data processor, such as a computing platform forexecuting a plurality of instructions. Optionally, the data processorincludes a volatile memory for storing instructions and/or data and/or anon-volatile storage, for example, a magnetic hard-disk and/or removablemedia, for storing instructions and/or data. Optionally, a networkconnection is provided as well. A display and/or a user input devicesuch as a keyboard or mouse are optionally provided as well.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIGS. 1A-G show results of computer simulations performed in accordancewith some embodiments of the present invention to analyze stressdistribution resulting from a crack in a modeling formulation;

FIG. 2 shows the effect of various concentrations of a formulation in acore on the HDT of the various printed objects;

FIG. 3 is a schematic illustration of an additive manufacturing systemaccording to some embodiments of the invention;

FIGS. 4A and 4B are schematic illustrations of a top view (FIG. 4A) anda side view (FIG. 4B) of additive manufacturing system employing rotarymotion, according to some embodiments of the invention;

FIG. 5 is a schematic illustration of an isometric view of an additivemanufacturing system employing rotary motion according to someembodiments of the invention;

FIGS. 6A-C are schematic illustrations of printing heads according tosome embodiments of the present invention;

FIG. 7 is a schematic illustration of an additive manufacturing systemin embodiments of the invention in which the system comprises a thermalscreen;

FIG. 8 is a graph showing a typical linear dependence of a temperatureon voltage applied to an infrared source;

FIGS. 9A-F are schematic illustrations of shelled structures, accordingto some embodiments of the present invention;

FIGS. 10A-B are schematic illustrations of an object formed on apedestal, according to some embodiments of the present invention;

FIG. 11 is a schematic illustration of a shelled structure having partsthat are devoid of a core region, according to some embodiments of thepresent invention;

FIG. 12 is a schematic illustration of a region which includesinterlaced modeling materials according to some embodiments of thepresent invention; and

FIGS. 13A-E show results of a dynamic mechanical analysis performed toanalyze an object fabricated according to some embodiments of thepresent invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

The present invention, in some embodiments thereof, relates to AdditiveManufacturing (AM) of an object, and, more particularly, but notexclusively, to methods and systems for additive manufacturing of alayered object.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the Examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways.

The method and system of the present embodiments manufacturethree-dimensional objects based on computer object data in a layerwisemanner by forming a plurality of layers in a configured patterncorresponding to the shape of the objects. The computer object data canbe in any known format, including, without limitation, a StandardTessellation Language (STL) or a StereoLithography Contour (SLC) format,Virtual Reality Modeling Language (VRML), Additive Manufacturing File(AMF) format, Drawing Exchange Format (DXF), Polygon File Format (PLY)or any other format suitable for Computer-Aided Design (CAD).

The term “object” as used herein refers to a whole object or a partthereof.

Each layer is formed by additive manufacturing apparatus which scans atwo-dimensional surface and patterns it. While scanning, the apparatusvisits a plurality of target locations on the two-dimensional layer orsurface, and decides, for each target location or a group of targetlocations, whether or not the target location or group of targetlocations is to be occupied by building material, and which type ofbuilding material is to be delivered thereto. The decision is madeaccording to a computer image of the surface.

In preferred embodiments of the present invention the AM comprisesthree-dimensional printing, more preferably three-dimensional inkjetprinting. In these embodiments a building material is dispensed from adispensing head having a circuit for controllably dispensing buildingmaterial in layers on a supporting structure. Typically, each dispensinghead optionally and preferably has a set of nozzles to deposit buildingmaterial in layers on a supporting structure. The AM apparatus thusdispenses building material in target locations which are to be occupiedand leaves other target locations void. The apparatus typically includesa plurality of dispensing heads, each of which can be configured todispense a different building material. Thus, different target locationscan be occupied by different building materials. The types of buildingmaterials can be categorized into two major categories: modelingmaterial and support material. The support material serves as asupporting matrix or construction for supporting the object or objectparts during the fabrication process and/or other purposes, e.g.,providing hollow or porous objects. Support constructions mayadditionally include modeling material elements, e.g. for furthersupport strength.

The modeling material is generally formed of a one or more formulationswhich are formulated for use in additive manufacturing and which areable to form, once cured, a three-dimensional object on its own, i.e.,without having to be mixed or combined with any other substance.

The final three-dimensional object is made of the modeling material or acombination of modeling materials or modeling and support materials ormodification thereof (e.g., following curing). All these operations arewell-known to those skilled in the art of solid freeform fabrication.

In some exemplary embodiments of the invention an object is manufacturedby dispensing a building material (uncured) which comprises two or moredifferent modeling material formulations, each formulation from adifferent dispensing head of the AM. The formulations are optionally andpreferably deposited in layers during the same pass of the dispensingheads. The formulations and combination of formulations within the layerare selected according to the desired properties of the object.

A representative and non-limiting example of a system 110 suitable forAM of an object 112 according to some embodiments of the presentinvention is illustrated in FIG. 3. System 110 comprises an additivemanufacturing apparatus 114 having a dispensing unit 16 which comprisesa plurality of dispensing heads. Each head preferably comprises an arrayof one or more nozzles 122, as illustrated in FIGS. 6A-C describedbelow, through which a liquid (uncured) building material 124 isdispensed.

Preferably, but not obligatorily, apparatus 114 is a three-dimensionalprinting apparatus, in which case the dispensing heads are printingheads, and the (uncured) building material is dispensed via inkjettechnology. This need not necessarily be the case, since, for someapplications, it may not be necessary for the additive manufacturingapparatus to employ three-dimensional printing techniques.Representative examples of additive manufacturing apparatus contemplatedaccording to various exemplary embodiments of the present inventioninclude, without limitation, fused deposition modeling apparatus andfused material deposition apparatus.

Each dispensing head is optionally and preferably fed via a buildingmaterial reservoir which may optionally include a temperature controlunit (e.g., a temperature sensor and/or a heating device), and amaterial level sensor. To dispense the (uncured) building material, avoltage signal is applied to the dispensing heads to selectively depositdroplets of material via the dispensing head nozzles, for example, as inpiezoelectric inkjet printing technology. The dispensing rate of eachhead depends on the number of nozzles, the type of nozzles and theapplied voltage signal rate (frequency). Such dispensing heads are knownto those skilled in the art of solid freeform fabrication.

Preferably, but not obligatorily, the overall number of dispensingnozzles or nozzle arrays is selected such that half of the dispensingnozzles are designated to dispense support material formulation and halfof the dispensing nozzles are designated to dispense modeling materialformulation(s), i.e. the number of nozzles jetting modeling materialformulation(s) is the same as the number of nozzles jetting supportmaterial formulation. In the representative example of FIG. 3, fourdispensing heads 16 a, 16 b, 16 c and 16 d are illustrated. Each ofheads 16 a, 16 b, 16 c and 16 d has a nozzle array. In this Example,heads 16 a and 16 b can be designated for modeling material/s and heads16 c and 16 d can be designated for support material. Thus, head 16 acan dispense a first modeling material formulation, head 16 b candispense a second modeling material formulation and heads 16 c and 16 dcan both dispense a support material formulation. In an alternativeembodiment, heads 16 c and 16 d, for example, may be combined in asingle head having two nozzle arrays for depositing a support materialformulation.

Yet it is to be understood that it is not intended to limit the scope ofthe present invention and that the number of modeling materialformulation depositing heads (modeling heads) and the number of supportmaterial formulation depositing heads (support heads) may differ.Generally, the number of modeling heads, the number of support heads andthe number of nozzles in each respective head or head array are selectedsuch as to provide a predetermined ratio, a, between the maximaldispensing rate of the support material formulation and the maximaldispensing rate of modeling material formulation(s). The value of thepredetermined ratio, a, is preferably selected to ensure that in eachformed layer, the height of dispensed modeling material formulation(s)equals the height of dispensed support material formulation(s). Typicalvalues for a are from about 0.6 to about 1.5.

As used herein throughout, the term “about” refers to ±10% or to ±5%.

For example, for a=1, the overall dispensing rate of a support materialformulation is generally the same as the overall dispensing rate of themodeling material formulation(s) when all modeling heads and supportheads operate.

In a preferred embodiment, there are M modeling heads each having marrays of p nozzles, and S support heads each having s arrays of qnozzles such that M×m×p=S×s×q. Each of the M×m modeling arrays and S×ssupport arrays can be manufactured as a separate physical unit, whichcan be assembled and disassembled from the group of arrays. In thisembodiment, each such array optionally and preferably comprises atemperature control unit and a material level sensor of its own, andreceives an individually controlled voltage for its operation.

Apparatus 114 can further comprise a hardening device 324 which caninclude any device configured to emit light, heat or the like that maycause the deposited material (dispensed formulations) to harden. Forexample, hardening device 324 can comprise one or more radiationsources, which can be, for example, an ultraviolet or visible orinfrared lamp, or other sources of electromagnetic radiation, orelectron beam source, depending on the modeling material formulationsand/or support material formulations being used. In some embodiments ofthe present invention, hardening device 324 serves for curing orsolidifying the curable material(s) in the formulation(s).

The dispensing head and radiation source are preferably mounted in aframe or block 128 which is preferably operative to reciprocally moveover a tray 360, which serves as the working surface. In someembodiments of the present invention the radiation sources are mountedin the block such that they follow in the wake of the dispensing headsto at least partially cure or solidify the curable materials justdispensed by the dispensing heads. Tray 360 is positioned horizontally.According to the common conventions an X-Y-Z Cartesian coordinate systemis selected such that the X-Y plane is parallel to tray 360. Tray 360 ispreferably configured to move vertically (along the Z direction),typically downward. In various exemplary embodiments of the invention,apparatus 114 further comprises one or more leveling devices 132, e.g. aroller 326. Leveling device 326 serves to straighten, level and/orestablish a thickness of the newly formed layer prior to the formationof the successive layer thereon. Leveling device 326 preferablycomprises a waste collection device 136 for collecting the excessmaterial generated during leveling. Waste collection device 136 maycomprise any mechanism that delivers the material to a waste tank orwaste cartridge.

In use, the dispensing heads of unit 16 move in a scanning direction,which is referred to herein as the X direction, and selectively dispense(uncured) building material in a predetermined configuration in thecourse of their passage over tray 360. The building material typicallycomprises one or more types of support material formulation(s) and oneor more types of modeling material formulation(s). The passage of thedispensing heads of unit 16 is followed by the curing of the dispensedmodeling material formulation(s) by radiation source 126. In the reversepassage of the heads, back to their starting point for the layer justdeposited, an additional dispensing of (uncured) building material maybe carried out, according to predetermined configuration. In the forwardand/or reverse passages of the dispensing heads, the layer thus formedmay be straightened by leveling device 326, which preferably follows thepath of the dispensing heads in their forward and/or reverse movement.Once the dispensing heads return to their starting point along the Xdirection, they may move to another position along an indexingdirection, referred to herein as the Y direction, and continue to buildthe same layer by reciprocal movement along the X direction.Alternately, the dispensing heads may move in the Y direction betweenforward and reverse movements or after more than one forward-reversemovement. The series of scans performed by the dispensing heads tocomplete a single layer is referred to herein as a single scan cycle.

Once the layer is completed, tray 360 is lowered in the Z direction to apredetermined Z level, according to the desired thickness of the layersubsequently to be printed. The procedure is repeated to formthree-dimensional object 112 in a layerwise manner.

In another embodiment, tray 360 may be displaced in the Z directionbetween forward and reverse passages of the dispensing head of unit 16,within the layer. Such Z displacement is carried out in order to causecontact of the leveling device with the surface in one direction andprevent contact in the other direction.

System 110 optionally and preferably comprises a building materialsupply system 330 which comprises the building material containers orcartridges and supplies a plurality of building materials to fabricationapparatus 114.

A control unit 340 controls fabrication apparatus 114 and optionally andpreferably also supply system 330. Control unit 340 typically includesan electronic circuit configured to perform the controlling operations.Control unit 340 preferably communicates with a data processor 154 whichtransmits digital data pertaining to fabrication instructions based oncomputer object data, e.g., a CAD configuration represented on acomputer readable medium in a form of a Standard Tessellation Language(STL) format or the like. Typically, control unit 340 controls thevoltage applied to each dispensing head or nozzle array and thetemperature of the building material in the respective printing head.

Once the manufacturing data is loaded to control unit 340 it can operatewithout user intervention. In some embodiments, control unit 340receives additional input from the operator, e.g., using data processor154 or using a user interface 116 communicating with unit 340. Userinterface 116 can be of any type known in the art, such as, but notlimited to, a keyboard, a touch screen and the like. For example,control unit 340 can receive, as additional input, one or more buildingmaterial types and/or attributes, such as, but not limited to, color,characteristic distortion and/or transition temperature, viscosity,electrical property, magnetic property. Other attributes and groups ofattributes are also contemplated.

Another representative and non-limiting example of a system 10 suitablefor AM of an object according to some embodiments of the presentinvention is illustrated in FIGS. 4A-B and 5. FIGS. 4A-B illustrate atop view (FIG. 4A) and a side view (FIG. 4B) and FIG. 5 illustrates anisometric view of system 10.

In the present embodiments, system 10 comprises a tray 12 and aplurality of inkjet printing heads 16, each having a plurality ofseparated nozzles. Tray 12 can have a shape of a disk or it can beannular. Non-round shapes are also contemplated, provided they can berotated about a vertical axis.

Tray 12 and heads 16 are optionally and preferably mounted such as toallow a relative rotary motion between tray 12 and heads 16. This can beachieved by (i) configuring tray 12 to rotate about a vertical axis 14relative to heads 16, (ii) configuring heads 16 to rotate about verticalaxis 14 relative to tray 12, or (iii) configuring both tray 12 and heads16 to rotate about vertical axis 14 but at different rotation velocities(e.g., rotation at opposite direction). While the embodiments below aredescribed with a particular emphasis to configuration (i) wherein thetray is a rotary tray that is configured to rotate about vertical axis14 relative to heads 16, it is to be understood that the presentapplication contemplates also configurations (ii) and (iii). Any one ofthe embodiments described herein can be adjusted to be applicable to anyof configurations (ii) and (iii), and one of ordinary skills in the art,provided with the details described herein, would know how to make suchadjustment.

In the following description, a direction parallel to tray 12 andpointing outwardly from axis 14 is referred to as the radial directionr, a direction parallel to tray 12 and perpendicular to the radialdirection r is referred to herein as the azimuthal direction φ, and adirection perpendicular to tray 12 is referred to herein is the verticaldirection z.

The term “radial position,” as used herein, refers to a position on orabove tray 12 at a specific distance from axis 14. When the term is usedin connection to a printing head, the term refers to a position of thehead which is at specific distance from axis 14. When the term is usedin connection to a point on tray 12, the term corresponds to any pointthat belongs to a locus of points that is a circle whose radius is thespecific distance from axis 14 and whose center is at axis 14.

The term “azimuthal position,” as used herein, refers to a position onor above tray 12 at a specific azimuthal angle relative to apredetermined reference point. Thus, radial position refers to any pointthat belongs to a locus of points that is a straight line forming thespecific azimuthal angle relative to the reference point.

The term “vertical position,” as used herein, refers to a position overa plane that intersect the vertical axis 14 at a specific point.

Tray 12 serves as a supporting structure for three-dimensional printing.The working area on which one or objects are printed is typically, butnot necessarily, smaller than the total area of tray 12. In someembodiments of the present invention the working area is annular. Theworking area is shown at 26. In some embodiments of the presentinvention tray 12 rotates continuously in the same direction throughoutthe formation of object, and in some embodiments of the presentinvention tray reverses the direction of rotation at least once (e.g.,in an oscillatory manner) during the formation of the object. Tray 12 isoptionally and preferably removable. Removing tray 12 can be formaintenance of system 10, or, if desired, for replacing the tray beforeprinting a new object. In some embodiments of the present inventionsystem 10 is provided with one or more different replacement trays(e.g., a kit of replacement trays), wherein two or more trays aredesignated for different types of objects (e.g., different weights)different operation modes (e.g., different rotation speeds), etc. Thereplacement of tray 12 can be manual or automatic, as desired. Whenautomatic replacement is employed, system 10 comprises a trayreplacement device 36 configured for removing tray 12 from its positionbelow heads 16 and replacing it by a replacement tray (not shown). Inthe representative illustration of FIG. 4A tray replacement device 36 isillustrated as a drive 38 with a movable arm 40 configured to pull tray12, but other types of tray replacement devices are also contemplated.

Exemplified embodiments for the printing head 16 are illustrated inFIGS. 6A-6C. These embodiments can be employed for any of the AM systemsdescribed above, including, without limitation, system 110 and system10.

FIGS. 6A-B illustrate a printing head 16 with one (FIG. 6A) and two(FIG. 6B) nozzle arrays 22. The nozzles in the array are preferablyaligned linearly, along a straight line. In embodiments in which aparticular printing head has two or more linear nozzle arrays, thenozzle arrays are optionally and preferably can be parallel to eachother.

When a system similar to system 110 is employed, all printing heads 16are optionally and preferably oriented along the indexing direction withtheir positions along the scanning direction being offset to oneanother.

When a system similar to system 10 is employed, all printing heads 16are optionally and preferably oriented radially (parallel to the radialdirection) with their azimuthal positions being offset to one another.Thus, in these embodiments, the nozzle arrays of different printingheads are not parallel to each other but are rather at an angle to eachother, which angle being approximately equal to the azimuthal offsetbetween the respective heads. For example, one head can be orientedradially and positioned at azimuthal position φ₁, and another head canbe oriented radially and positioned at azimuthal position φ₂. In thisexample, the azimuthal offset between the two heads is φ₁-φ₂, and theangle between the linear nozzle arrays of the two heads is also φ₁-φ₂.

In some embodiments, two or more printing heads can be assembled to ablock of printing heads, in which case the printing heads of the blockare typically parallel to each other. A block including several inkjetprinting heads 16 a, 16 b, 16 c is illustrated in FIG. 6C.

In some embodiments, system 10 comprises a support structure 30positioned below heads 16 such that tray 12 is between support structure30 and heads 16. Support structure 30 may serve for preventing orreducing vibrations of tray 12 that may occur while inkjet printingheads 16 operate. In configurations in which printing heads 16 rotateabout axis 14, support structure 30 preferably also rotates such thatsupport structure 30 is always directly below heads 16 (with tray 12between heads 16 and tray 12).

Tray 12 and/or printing heads 16 is optionally and preferably configuredto move along the vertical direction z, parallel to vertical axis 14 soas to vary the vertical distance between tray 12 and printing heads 16.In configurations in which the vertical distance is varied by movingtray 12 along the vertical direction, support structure 30 preferablyalso moves vertically together with tray 12. In configurations in whichthe vertical distance is varied by heads 16 along the verticaldirection, while maintaining the vertical position of tray 12 fixed,support structure 30 is also maintained at a fixed vertical position.

The vertical motion can be established by a vertical drive 28. Once alayer is completed, the vertical distance between tray 12 and heads 16can be increased (e.g., tray 12 is lowered relative to heads 16) by apredetermined vertical step, according to the desired thickness of thelayer subsequently to be printed. The procedure is repeated to form athree-dimensional object in a layerwise manner.

The operation of inkjet printing heads 16 and optionally and preferablyalso of one or more other components of system 10, e.g., the motion oftray 12, are controlled by a controller 20. The controller can has anelectronic circuit and a non-volatile memory medium readable by thecircuit, wherein the memory medium stores program instructions which,when read by the circuit, cause the circuit to perform controloperations as further detailed below.

Controller 20 can also communicate with a host computer 24 whichtransmits digital data pertaining to fabrication instructions based oncomputer object data, e.g., in a form of a Standard TessellationLanguage (STL) or a StereoLithography Contour (SLC) format, VirtualReality Modeling Language (VRML), Additive Manufacturing File (AMF)format, Drawing Exchange Format (DXF), Polygon File Format (PLY) or anyother format suitable for Computer-Aided Design (CAD). The object dataformats are typically structured according to a Cartesian system ofcoordinates. In these cases, computer 24 preferably executes a procedurefor transforming the coordinates of each slice in the computer objectdata from a Cartesian system of coordinates into a polar system ofcoordinates. Computer 24 optionally and preferably transmits thefabrication instructions in terms of the transformed system ofcoordinates. Alternatively, computer 24 can transmit the fabricationinstructions in terms of the original system of coordinates as providedby the computer object data, in which case the transformation ofcoordinates is executed by the circuit of controller 20.

The transformation of coordinates allows three-dimensional printing overa rotating tray. In conventional three-dimensional printing, theprinting heads reciprocally move above a stationary tray along straightlines. In such conventional systems, the printing resolution is the sameat any point over the tray, provided the dispensing rates of the headsare uniform. Unlike conventional three-dimensional printing, not all thenozzles of the head points cover the same distance over tray 12 duringat the same time. The transformation of coordinates is optionally andpreferably executed so as to ensure equal amounts of excess material atdifferent radial positions.

Typically, controller 20 controls the voltage applied to the respectivecomponent of the system 10 based on the fabrication instructions andbased on the stored program instructions as described below.

Generally, controller 20 controls printing heads 16 to dispense, duringthe rotation of tray 12, droplets of building material in layers, suchas to print a three-dimensional object on tray 12.

System 10 optionally and preferably comprises one or more radiationsources 18, which can be, for example, an ultraviolet or visible orinfrared lamp, or other sources of electromagnetic radiation, orelectron beam source, depending on the modeling material being used.Radiation source can include any type of radiation emitting device,including, without limitation, light emitting diode (LED), digital lightprocessing (DLP) system, resistive lamp and the like. Radiation source18 serves for curing or solidifying or hardening the dispensed modelingmaterial formulation(s) and/or support material formulation(s). Invarious exemplary embodiments of the invention the operation ofradiation source 18 is controlled by controller 20 which may activateand deactivate radiation source 18 and may optionally also control theamount of radiation generated by radiation source 18.

In some embodiments of the invention, system 10 further comprises one ormore leveling devices 32 which can be manufactured as a roller or ablade. Leveling device 32 serves to straighten the newly formed layerprior to the formation of the successive layer thereon. In someembodiments, leveling device 32 has the shape of a conical rollerpositioned such that its symmetry axis 34 is tilted relative to thesurface of tray 12 and its surface is parallel to the surface of thetray. This embodiment is illustrated in the side view of system 10 (FIG.4B).

The conical roller can have the shape of a cone or a conical frustum.

The opening angle of the conical roller is preferably selected such thatthere is a constant ratio between the radius of the cone at any locationalong its axis 34 and the distance between that location and axis 14.This embodiment allows roller 32 to efficiently level the layers, sincewhile the roller rotates, any point p on the surface of the roller has alinear velocity which is proportional (e.g., the same) to the linearvelocity of the tray at a point vertically beneath point p. In someembodiments, the roller has a shape of a conical frustum having a heighth, a radius R₁ at its closest distance from axis 14, and a radius R₂ atits farthest distance from axis 14, wherein the parameters h, R₁ and R₂satisfy the relation R₁/R₂=(R−h)/h and wherein R is the farthestdistance of the roller from axis 14 (for example, R can be the radius oftray 12).

The operation of leveling device 32 is optionally and preferablycontrolled by controller 20 which may activate and deactivate levelingdevice 32 and may optionally also control its position along a verticaldirection (parallel to axis 14) and/or a radial direction (parallel totray 12 and pointing toward or away from axis 14.

In some embodiments of the present invention printing heads 16 areconfigured to reciprocally move relative to tray along the radialdirection r. These embodiments are useful when the lengths of the nozzlearrays 22 of heads 16 are shorter than the width along the radialdirection of the working area 26 on tray 12. The motion of heads 16along the radial direction is optionally and preferably controlled bycontroller 20.

Some embodiments contemplate the fabrication of an object by dispensingdifferent formulations, comprising, e.g., different curable materials,from different dispensing heads. These embodiments provide, inter alia,the ability to select curable materials or combinations of curablematerials from a given number of materials and define desiredcombinations of the selected curable materials and their properties.According to the present embodiments, the spatial locations of thedeposition of each curable material or combinations of curable materialswith the layer is defined, either to effect occupation of differentthree-dimensional spatial locations by different materials or differentcombinations of materials, or to effect occupation of substantially thesame three-dimensional location or adjacent three-dimensional locationsby two or more different curable materials or different combinations ofcurable materials so as to allow post deposition spatial combination ofthe materials within the layer, thereby to form a composite material atthe respective location or locations.

Any post deposition combination or mix of modeling material formulationsis contemplated. For example, once a certain modeling materialformulation is dispensed it may preserve its original properties.However, when it is dispensed simultaneously with another modelingmaterial formulation or other dispensed (curable) materials which aredispensed at the same or nearby locations, a composite material having adifferent property or properties to the dispensed materials is formed.

The present embodiments thus enable the deposition of a broad range ofmaterial combinations, and the fabrication of an object which mayconsist of multiple different combinations of materials, in differentparts of the object, according to the properties desired to characterizeeach part of the object.

Further details on the principles and operations of an AM systemsuitable for the present embodiments are found in U.S. PublishedApplication No. 20100191360, and International Publication No.WO2016/009426, the contents of which are hereby incorporated byreference.

The systems of the present embodiments (system 10 and system 110) areoptionally and preferably supplemented with a thermal screen forthermally separating the circuits of the dispensing or printing headsfrom the space between the heads and the tray. A representativeschematic illustration of this embodiment is shown in FIG. 7. FIG. 7illustrates the system for the case of linear relative motion betweenthe tray and the heads, but the ordinarily skilled person, provided withthe details described herein, would know how to adjust the drawing forthe case in which the relative motion is rotary (for example, byreplacing tray 360 of with tray 12, and printing block 128 by at leastone of printing head 16, radiation source 18 and leveling device 32).For clarity of presentation, several features of the systems such as thesupply apparatus, the user interface and the controller have beenomitted from FIG. 7.

Shown in FIG. 7 is a printing chamber 700 having therein the tray 360and the printing block 128. Block 128 represents all the elements thatare used for dispensing and hardening the building materials, including,without limitation, the printing heads, leveling devices and hardeningdevices. The electronic circuits of block 128 (e.g., the electroniccircuit of the printing heads, leveling devices and/or hardeningdevices) are located at the upper part of block 128 and are collectivelyshown at 704. The lower part of block 128 can include the nozzles 122 ofthe printing heads, the mechanical parts of leveling device 326 and theoutput of the hardening device 324).

A thermal screen 702 separates between the upper part 700 a and lowerpart 700 b of chamber 700, such that the upper part of block 128,including electronic circuits 704, is above thermal screen 702, and thelower part of block 128 is below thermal screen 702. This ensures thatthe electronic circuits 704 are thermally separated from the componentsof block 128 that dispense or otherwise interact (mechanically or by wayof radiation) with the building material. This embodiment is useful whenthe system is used to dispense the first and/or second modelingformulations described herein, particularly when heat is applied to theformulations, for example, for reducing the curling effect.

Thermal screen 702 is optionally and preferably collapsible andexpandable, and is positioned to simultaneously fold at one side ofblock 128 and expand at an opposite side of block 128 during the motionof block 128. For example, screen 702 can be structured as an accordionfolding screen or as a telescopic screen, e.g., a concentric coupling ofa series of nested and axially interlocking hollow structures ofgradually reducing dimensions. Screen 702 can be made of, or coated by,a thermally reflective material.

In some embodiments of the present invention system 110 (or system 10)comprises a heating system 706 that heats the lower part 700 b ofchamber 700, particularly the space between the printing head and thetray. Heating system 706 can be embodied in more than one way. In someembodiments of the present invention, heating system 706 comprises atray heater 708 in thermal contact with a back side of tray 360 fordelivering heat to the modeling material that is dispensed on the frontside of the tray by heat conduction.

Tray heater 708 can comprises one or more heating panels havingresistance filament. When tray heater 708 is employed, tray 360 is madeof a heat conductive material, such as a metal, e.g., aluminum.Typically, but not necessarily, the resistance filament can be coated byor embedded in an encapsulation, such as, but not limited to, a siliconencapsulation or the like. The heating panel(s) are preferably disposedso as to cover the entire back side of the working area of tray 360. Thetemperature of heater 708 can be controlled by a temperature controlcircuit 714, such as, but not limited to, aproportional-integral-derivative (PID) controller. Temperature controlcircuit 714 can receive temperature data from a temperature sensor 716,such as, but not limited to, a thermocouple, positioned in contact withheater 708 and control the voltage on the resistance filamentresponsively to the received temperature data and to control signalsreceived from the main controller (not shown, see 152 in FIGS. 3 and 20in FIG. 4A).

Typical operational parameters of tray heater 708 are, withoutlimitation, temperature range of 40-100° C., maximal flux about 1-2w/cm², e.g., about 1.5 w/cm², maximal applied voltage about 360-400volts, e.g., about 380 volts, or from about 150 volts to about 230volts.

In some embodiments of the present invention heating system 706comprises a radiation source 718 that delivers heat to the dispensedmodeling material formulation(s) by radiation (e.g., infraredradiation). Radiative heat is optionally and preferably applied from thetop side of the tray so as to allow heating dispensed material that isfarther from the tray. For example, source 718 can be mounted on block128 (e.g., alongside device 324), so as to allow it to move reciprocallyover the tray. The radiation source 718 can be controlled by atemperature control circuit 720, such as, but not limited to, a (PID)controller, which receives temperature data from a temperature sensor(not shown) that is mounted on the source 718, and provides voltagepulses to the source responsively to the received temperature data andto control signals received from the main controller (not shown, see 152in FIGS. 3 and 20 in FIG. 4A). Alternatively, an open loop control canbe employed, in which case a constant voltage level is applied to thesource without dynamically controlling the voltage based on temperaturedata.

Typical operational parameters of infrared source 718 are, withoutlimitation, temp range of 40-900° C., wavelength range 2-10 μm, maximalflux 6-7 w/cm², e.g., about 6.4 w/cm², voltage 150-400 volts, e.g.,about 180 volts. FIG. 8 is a graph showing a typical linear dependenceof the temperature inside infrared source 718 as a function of thevoltage applied to source 718.

In some embodiments of the present invention, heating system 706comprises a chamber heater 712 for delivering heat to the modelingmaterial that is dispensed on the front side of the tray by heatconduction. In some embodiments of the present invention, heating system706 comprises a blower and/or fan 710 positioned outside the spacebetween the block 128 and the tray 360 (e.g., below the tray) fordelivering heat to the dispensed modeling material by convection. Heatconvection (e.g., by air) is generally shown by block arrows. Use ofchamber heater 712 optionally and preferably in combination with blowerand/or fan 710 is advantageous because it allows heating also the sidewalls and the top of the printed object. Preferably, the chamber heater712 is activated before (e.g., 10-60 minutes before) the dispensing ofbuilding material is initiated.

The chamber heater 712 and/or blower and/or fan 710 is or can becontrolled by a temperature control circuit 722, such as, but notlimited to, a (PID) controller, which receives temperature data from atemperature sensor 724 that is mounted in the space between block 128and tray 360, and controls the temperature of chamber heater 712 and/orthe fan speed of blower or fan 710 responsively to the receivedtemperature data and to control signals received from the maincontroller (not shown, see 152 in FIGS. 3 and 20 in FIG. 4A). Themaximal temperature of heater 712 is, without limitation 600-700° C.,e.g., about 650° C., and the maximal air flow generated by blower or fan710 is, without limitation 250-350 l/min, e.g., about 300 l/min.

Preferably, heating system 706 includes two or more of the aboveelements, so as to allow system 706 to deliver heat by at least twomechanisms selected from the group consisting of heat conduction, heatconvection and radiation. In some embodiments of the present inventionthe controller (not shown, see 152 in FIGS. 3 and 20 in FIG. 4A)receives from the user interface a heating mode and operates heatingsystem according to received mode. The mode can be selected from apredetermined list heating modes. For example, in one heating mode, thetray heater, infrared radiation, chamber heater and blower or fan areoperated. In another heating mode, the tray heater, infrared radiationand chamber heater are operated, but the blower or fan is not operated.In another heating mode, the tray heater and infrared radiation areoperated, but the chamber heater and blower or fan is not operated. Inanother heating mode, the tray heater, infrared radiation and blower orfan are operated, but the chamber heater is not operated. In anotherheating mode, the tray heater and chamber heater is operated, but theinfrared radiation and blower or fan are not operated. In anotherheating mode, the tray heater, chamber heater is operated and blower orfan are operated, but the infrared radiation is not operated. Typically,but not necessarily the user interface displays this list of modes tothe user and allows the user to select the desired mode.

Even though inkjet printing is widely practiced and has become a routineprocedure for fabricating arbitrarily shaped structures throughout theworld, it is not without certain operative limitations. For example, therange of thermo-mechanical properties obtainable from currentlyavailable modeling materials can be insufficient, in particular forthose modeling materials which are related to UV polymerization, andwhich are formed of low molecular weight raw materials (e.g., monomersand oligomers), and especially if the raw materials polymerize by aradical mechanism, e.g., the addition reaction of acrylic functionalgroups.

The present inventors have devised a layered (e.g., polymeric) object orstructure which enjoys thermo-mechanical properties which are improvedcompared to other objects fabricated via AM.

Generally, the structure according to various exemplary embodiments ofthe present invention is a layered shelled structure made of two or moremodeling formulations (e.g., UV-polymerizable or UV-curable modelingformulations). The structure typically comprises a layered core which isat least partially coated by one or more layered shells such that atleast one layer of the core engages the same plane with a layer of atleast one of the shells. The thickness of each shell, as measuredperpendicularly to the surface of the structure, is typically, but notnecessarily, at least 10 μm. In various exemplary embodiments of theinvention, the core and the shell are different from each other in theirthermo-mechanical properties. This is readily achieved by fabricatingthe core and shell from different modeling formulations or differentcombinations of modeling formulations. The thermo-mechanical propertiesof the core and shell are referred to herein as “core thermo-mechanicalproperties” and “shell thermo-mechanical properties,” respectively.

A representative and non-limiting example of a structure according tosome embodiments of the present invention is shown in FIGS. 9A-D.

FIG. 9A is a schematic illustration of a perspective view of a structure60, and FIG. 9B is a cross-sectional view of structure 60 along line A-Aof FIG. 9A. For clarity of presentation a Cartesian coordinate system isalso illustrated.

Structure 60 comprises a plurality of layers 62 stacked along the zdirection. Structure 60 is typically fabricated by an AM technique,e.g., using system 10, whereby the layers are formed in a sequentialmanner. Thus, the z direction is also referred to herein as the “builddirection” of the structure. Layers 62 are, therefore, perpendicular tothe build direction. Although structure 60 is shown as a cylinder, thisneed not necessarily be the case, since the structure of the presentembodiments can have any shape.

The shell and core of structure 60 are shown at 64 and 66, respectively.As shown, the layers of core 66 and the layers of shell 64 areco-planar. The AM technique allows the simultaneous fabrication of shell64 and core 66, whereby for a particular formed layer, the inner part ofthe layer constitutes a layer of the core, and the periphery of thelayer, or part thereof, constitutes a layer of the shell.

A peripheral section of a layer which contributes to shell 64 isreferred to herein as an “envelope region” of the layer. In thenon-limiting example of FIGS. 9A and 9B, each of layers 62 has anenvelope region. Namely, each layer in FIGS. 9A and 9B contributes bothto the core and to the shell. However, this need not necessarily be thecase, since, for some applications, it may be desired to have the coreexposed to the environment in some regions. In these applications, atleast some of the layers do not include an envelope region. Arepresentative example of such configuration is illustrated in thecross-sectional view of FIG. 9C, showing some layers 68 which contributeto the core but not to the shell, and some layers 70 which contribute toboth the core and the shell. In some embodiments, one or more layers donot include a region with core thermo-mechanical properties and compriseonly a region with shell thermo-mechanical properties. These embodimentsare particularly useful when the structure has one or more thin parts,wherein the layers forming those parts of the structure are preferablydevoid of a core region. A representative example of such a structure isillustrated in FIG. 11 described below. Also contemplated areembodiments in which one or more layers do not include a region withshell thermo-mechanical properties and comprise only a region with corethermo-mechanical properties.

The shell can, optionally and preferably, also cover structure 60 fromabove and/or below, relative to the z direction. In these embodiments,some layers at the top most and/or bottom most parts of structure 60have at least one material property which is different from core 66. Invarious exemplary embodiments of the invention the top most and/orbottom most parts of structure 60 have the same material property asshell 64. A representative example of this embodiment is illustrated inFIG. 9D. The top/bottom shell of structure 60 may be thinner (e.g., 2times thinner) than the side shell, e.g. when the top or bottom shellcomprises a layer above or below the structure, and therefore has thesame thickness as required for layers forming the object.

A representative example of a layer 62 suitable for some embodiments ofthe present invention is illustrated in FIG. 9E. In the schematicillustration of FIG. 9E, which is not to be considered as limiting,layer 62 has a core region 902, an inner envelope region 904, at leastpartially, more preferably completely, surrounding core region 902, andan outer envelope region 906, at least partially, more preferablycompletely, surrounding inner envelope region 904. Preferably, but notnecessarily, outer envelope region 906 is the outermost region of layer62.

Core region 902 preferably comprises a combination of at least twomodeling formulations. The combination is optionally and preferablyembodied in a voxelated manner wherein some voxels that form region 902are made of one of the modeling material formulations, other voxels aremade of another one of the modeling material formulations, and so on. Invarious exemplary embodiments of the invention core region 902 is madeof a voxelated combination between a first modeling formulation and asecond modeling formulation as described below. The voxelatedcombination can be according to any distribution by which voxelsoccupied by the first formulation are interlaced within voxels occupiedby the second formulation, such as, but not limited to, a randomdistribution.

The ratio between the number of voxels within region 902 that areoccupied by a first modeling formulation and the number of voxels withinregion 902 that are occupied by a second modeling formulation isoptionally from about 0.25 to about 0.45, or from about 0.25 to about0.4, or from about 0.3 to about 0.4, e.g., about 0.33. Other ratios,e.g., ratios in which the number of voxels within region 902 that areoccupied by the first modeling formulation is equal to or larger (e.g.,1.5 times larger or 2 times larger or 2.5 times larger or 3 timeslarger) than the number of voxels within region 902 that are occupied bythe second modeling are also contemplated in some embodiments. In anyembodiment of the invention, including any embodiment that includes theabove ratios, region 902 is optionally and preferably devoid of anymaterial other than the first formulation and the second formulationdescribed herein.

Further embodiments related to the ratio between the first modelingmaterial formulation and the second modeling material formulation areprovided hereinunder.

Inner envelope region 904 is preferably made of a single modelingformulation, for example, the first modeling formulation describedbelow. Outer envelope region 906 is preferably made of a single modelingformulation, for example, the second modeling formulation describedbelow.

The thickness of region 904, as measured within the plane of layer 62and perpendicularly to the surface of structure 60, is preferably fromabout 0.1 mm to about 4 mm, or from about 0.1 mm to about 3.5 mm, orfrom about 0.1 mm to about 3 mm, or from about 0.1 mm to about 2.5 mm,or from about 0.1 mm to about 2 mm, or from about 0.2 mm to about 1.5mm, or from about 0.3 mm to about 1.5 mm, or from about 0.4 mm to about1.5 mm, or from about 0.4 mm to about 1.4 mm or from about 0.4 mm toabout 1.3 mm or from about 0.4 mm to about 1.2 mm or from about 0.4 mmto about 1.1 mm. The thickness of region 906, as measured within theplane of layer 62 and perpendicularly to the surface of structure 60, ispreferably from about from about 150 microns to about 600 microns, orfrom about from about 150 microns to about 550 microns, or from aboutfrom about 150 microns to about 500 microns, or from about from about150 microns to about 450 microns, or from about from about 150 micronsto about 400 microns, or from about from about 150 microns to about 350microns, or from about 180 microns to about 320 microns, or from about200 microns to about 300 microns, or from about 220 microns to about 280microns, or from about 240 microns to about 260 microns.

In some embodiments of the present invention, layer 62 comprises anadditional envelope region 908 between inner envelope region 904 andouter envelope region 906. Region 908 is preferably made of acombination, e.g., voxelated combination, of two or more modelingformulations. Typically, but not exclusively, region 908 is made of avoxelated combination including the modeling formulation making region904 (the first modeling formulation in the above example) and themodeling formulation making region 906 (the second modeling formulationin the above example). It was found by the Inventors of the presentinvention that such configuration allows region 908 to serve as astitching region that bonds region 906 to region 904.

The ratio between the number of voxels within region 908 that areoccupied by the first modeling formulation and the number of voxelswithin region 902 that are occupied by the second modeling formulationis preferably from about 0.9 to about 1.1, e.g., about 1. In anyembodiment of the invention, including any embodiment that includesthese ratios, region 908 is optionally and preferably devoid of anymaterial other than the first formulation and the second formulationdescribed herein. The thickness of region 908, as measured within theplane of layer 62 and perpendicularly to the surface of structure 60, ispreferably less than the thickness of region 904 and also less than thethickness of region 906. For example, the thickness of region 908 can befrom about 70 microns to about 100 microns or from about 75 microns toabout 95 microns or from about 80 microns to about 90 microns.

In some embodiments, one or more layers do not include a core region andcomprise only envelope regions. These embodiments are particularlyuseful when the structure has one or more thin parts, wherein the layersforming those parts of the structure are preferably devoid of a coreregion. A representative example of such a structure is illustrated inFIG. 11, in which regions marked by dashed circles are devoid of core902.

FIG. 9F is a schematic illustration of a side view of structure 60 inembodiments of the invention in which at least some of the layers 62 ofstructure 60 comprise core region 902, envelope regions 904 and 906 andoptionally also an additional envelope region 908 between regions 904and 906. In these embodiments structure 60 optionally and preferablycomprises a base section 910 and/or a top section 920, each optionallyand preferably comprises a plurality of layers.

The layers of sections 910 and 920 can be arranged such that one or moreof the topmost layers 922 of top section 920 and one or more of thebottommost layers 912 of base section 910 are made of the sameformulation at envelope region 906 described above. Alternatively, ormore preferably additionally, the layers of sections 910 and 920 can bearranged such that one or more of the bottommost layers 924 of topsection 920 and one or more of the topmost layers 914 of base section910 are made of the same formulation at envelope region 904 describedabove. In some embodiments of the present invention at least one of basesection 910 and top section 920 comprises one or more intermediatelayers (respectively shown at 918, 928) that is made of the same orsimilar combination of formulations as region 908 described above.

For clarity of presentation, FIG. 9F shows a single layer for each oflayers 912, 914, 918, 922, 924 and 928, however, this need notnecessarily be the case, since, for some applications, at least one ofthese layers is embodied as a stack of layers. The number of layers ineach stack is preferably selected such that the thickness, along thebuild direction (the z direction, in the present illustration) of thestack is a proximately the same as the thickness of the respectiveenvelope region. Specifically, the number of layers in stacks 912 and922 is preferably selected such that the overall thickness of thesestacks along the build direction is approximately the same (e.g., within10%) as the thickness of outer envelope region 906 as measured in theplane of layer 62 and perpendicularly to the surface of structure 60,the number of layers in stacks 914 and 924 is preferably selected suchthat the overall thickness of these stacks along the build direction isapproximately the same (e.g., within 10%) as the thickness of innerenvelope region 904 as measured in the plane of layer 62 andperpendicularly to the surface of structure 60, and the number of layersin stacks 918 and 928 is preferably selected such that the overallthickness of these stacks along the build direction is approximately thesame (e.g., within 10%) as the thickness of additional envelope region908 as measured in the plane of layer 62 and perpendicularly to thesurface of structure 60,

The present embodiments thus provide a method of layerwise fabricationof a three-dimensional object, in which for each of at least a few(e.g., at least two or at least three or at least 10 or at least 20 orat least 40 or at least 80) of the layers or all the layers, a buildingmaterial comprising two or more modeling formulations is dispensed,optionally and preferably using system 10 or system 110. The buildingmaterial, according to the present embodiments, comprises at least afirst modeling material formulation and a second modeling materialformulation, which are dispensed so as to form a core region using boththe first and the second modeling material formulations, an innerenvelope region at least partially surrounding the core region using thefirst modeling formulation but not the second modeling formulation, andan outer envelope region at least partially surrounding the innerenvelope region using the second modeling formulations but not the firstmodeling formulation, as described herein in any of the respectiveembodiments. Each modeling formulation is preferably dispensed byjetting it out of a plurality of nozzles of a print head (e.g., printhead 16). The dispensing is optionally and preferably in a voxelatedmanner.

The core region is optionally and preferably formed from a firstmodeling formulation as well as a second modeling formulation, asdescribed herein in any of the respective embodiments. This isoptionally and preferably, but not necessarily, achieved by interlacingvoxels of the first modeling formulation and voxels of the secondmodeling formulation within the core according to a predetermined voxelratio. In some embodiments of the present invention the amount of thefirst modeling formulation in the core region is 25%, or higher than 25%or higher than 26% or higher than 27% or higher than 28% or higher than29% or higher than 30% of a total weight of core region. In someembodiments of the present invention the ratio between the weight of thefirst modeling formulation in the core region and the weight of thesecond modeling formulation in the core region is from about 0.1 toabout 10, or from about 0.2 to about 5, or from about 0.2 to about 2, orfrom about 0.2 to about 1, or from about 0.2 to about 0.5, or from about1 to about 10, or from about 2 to about 10, or from about 5 to about 10.

Once formed, the layer including the two modeling formulations ispreferably exposed to a curing condition (e.g., curing energy) so as toharden the formulations. This is optionally and preferably executedusing hardening device 324 or radiation source 18. Alternatively, acuring condition can be exposure to the environment and/or to a chemicalreagent.

In some of any of the embodiments described herein, the buildingmaterial further comprises a support material formulation.

In some of any of the embodiments described herein, dispensing abuilding material formulation (uncured building material) furthercomprises dispensing support material formulation(s) which form thesupport material upon application of curing energy.

Dispensing the support material formulation, in some embodiments, iseffected by inkjet printing head(s) other than the inkjet printing headsused for dispensing the first and second (and other) compositionsforming the modeling material.

In some embodiments, exposing the dispensed building material to acuring condition (e.g., curing energy) includes applying a curingcondition (e.g., curing energy) that affects curing of a supportmaterial formulation, to thereby obtain a cured support material.

In some of any of the embodiments described herein, once a dispensedbuilding material is cured, the method further comprises removing thecured support material. Any of the methods usable for removing a supportmaterial can be used, depending on the materials forming the modelingmaterial and the support material. Such methods include, for example,mechanical removal of the cured support material and/or chemical removalof the cured support material by contacting the cured support materialwith a solution in which it is dissolvable (e.g., an alkaline aqueoussolution).

As used herein, the term “curing” describes a process in which aformulation is hardened or solidifies, and is also referred to herein as“hardening”. This term encompasses polymerization of monomer(s) and/oroligomer(s) and/or cross-linking of polymeric chains (either of apolymer present before curing or of a polymeric material formed in apolymerization of the monomers or oligomers). This term alternativelyencompasses solidification of the formulation that does not involvepolymerization and/or cross-linking.

The product of a curing reaction is typically a polymeric material andin some cases a cross-linked polymeric material. This term, as usedherein, encompasses also partial curing, for example, curing of at least20% or at least 30% or at least 40% or at least 50% or at least 60% orat least 70% of the formulation, as well as 100% of the formulation.

A “curing energy” typically includes application of radiation orapplication of heat, as described herein.

A curable material or formulation that undergoes curing upon exposure toelectromagnetic radiation is referred to herein interchangeably as“photopolymerizable” or “photoactivatable” or “photocurable”.

A curable material or formulation that undergoes curing upon exposure toUV radiation is referred to herein interchangeably as “UV-curable” or“UV-polymerizable”.

When the curing energy comprises heat, the curing is also referred toherein and in the art as “thermal curing” and comprises application ofthermal energy. Applying thermal energy can be effected, for example, byheating a receiving medium onto which the layers are dispensed or achamber hosting the receiving medium, as described herein. In someembodiments, the heating is effected using a resistive heater.

In some embodiments, the heating is effected by irradiating thedispensed layers by heat-inducing radiation. Such irradiation can beeffected, for example, by means of an IR lamp or Xenon lamp, operated toemit radiation onto the deposited layer.

In some of any of the embodiments described herein, the method furthercomprises exposing the cured or solidified modeling material, eitherbefore or after removal of a cured support material, if a supportmaterial formulation has been included in the building material, to apost-treatment condition. The post-treatment condition is typicallyaimed at further hardening the cured modeling material. In someembodiments, the post-treatment hardens a partially-cured material tothereby obtain a completely cured material.

In some embodiments, the post-treatment is effected by exposure to heator radiation, as described in any of the respective embodiments herein.In some embodiments, when the condition is heat, the post-treatment canbe effected for a time period that ranges from a few minutes (e.g., 10minutes) to a few hours (e.g., 1-24 hours).

The post-treatment condition is also referred to herein as posthardening treatment of post curing treatment.

In some embodiments of the present invention, the layer, once formed andhardened, is subjected to a post hardening treatment. Preferably, thepost hardening treatment is a thermal treatment, more preferablyheating. In a preferred embodiment, the post curing treatment includesmaintaining a temperature of at least 120° C., for a time period of atleast 1 hour.

The term “post-treatment” is also referred to herein interchangeably as“post-curing treatment” or simply as “post-curing”, or as“post-hardening treatment”.

In some embodiments of the present invention the layers are exposed toheat, during the dispensing of the formulation and/or during theexposure to curing energy. This can be executed using heating system706. The heating is preferably to a temperature which is below the HDTof the first modeling formulation, for example, at least 10° C. belowthe HDT of the first formulation. The heating can be to a temperaturewhich above the HDT of the second modeling formulation. More preferably,the heating is to a temperature which is below (e.g., at least 10° C.below) the HDT of the first modeling formulation and above an HDT ofsecond modeling formulation.

Typical temperatures to which the layer is heated, including, withoutlimitation, at least 40° C., or from about 40° C. to about 60° C.

According to some of any of the embodiments described herein, each ofthe modeling material formulations comprises one or more curablematerials.

Herein throughout, a “curable material” or a “solidifiable material” isa compound (e.g., monomeric or oligomeric or polymeric compound) which,when exposed to curing condition (e.g., curing energy), as describedherein, solidifies or hardens to form a cured modeling material asdefined herein. Curable materials are typically polymerizable materials,which undergo polymerization and/or cross-linking when exposed tosuitable energy source.

In some of any of the embodiments described herein, a curable materialis a photopolymerizable material, which polymerizes or undergoescross-linking upon exposure to radiation, as described herein, and insome embodiments the curable material is a UV-curable material, whichpolymerizes or undergoes cross-linking upon exposure to UV-visradiation, as described herein.

In some embodiments, a curable material as described herein is apolymerizable material that polymerizes via photo-induced radicalpolymerization.

In some of any of the embodiments described herein, a curable materialcan be a monomer, an oligomer or a short-chain polymer, each beingpolymerizable as described herein.

In some of any of the embodiments described herein, when a curablematerial is exposed to curing energy (e.g., radiation), it polymerizesby any one, or combination, of chain elongation and cross-linking.

In some of any of the embodiments described herein, a curable materialis a monomer or a mixture of monomers which can form a polymericmodeling material upon a polymerization reaction, when exposed to curingenergy at which the polymerization reaction occurs. Such curablematerials are also referred to herein as monomeric curable materials.

In some of any of the embodiments described herein, a curable materialis an oligomer or a mixture of oligomers which can form a polymericmodeling material upon a polymerization reaction, when exposed to curingenergy at which the polymerization reaction occurs. Such curablematerials are also referred to herein as oligomeric curable materials.

In some of any of the embodiments described herein, a curable materialis a mixture of one or more monomers and one or more oligomers which canform a co-polymeric modeling material upon a polymerization reaction,when exposed to curing energy at which the polymerization reactionoccurs.

In some of any of the embodiments described herein, a curable material,whether monomeric and/or oligomeric, can be a mono-functional curablematerial or a multi-functional curable material.

Herein, a mono-functional curable material comprises one functionalgroup that can undergo polymerization when exposed to curing energy(e.g., radiation).

A multi-functional curable material comprises two or more, e.g., 2, 3, 4or more, functional groups that can undergo polymerization when exposedto curing energy. Multi-functional curable materials can be, forexample, di-functional, tri-functional or tetra-functional curablematerials, which comprise 2, 3 or 4 groups that can undergopolymerization, respectively. The two or more functional groups in amulti-functional curable material are typically linked to one another bya linking moiety. When the linking moiety is an oligomeric moiety, themulti-functional group is an oligomeric multi-functional curablematerial.

Multi-functional curable materials typically effect cross-linking, forexample, of polymeric chains formed of mono-functional ormulti-functional curable materials, and can therefore function ascross-linking agents. It is to be noted that curable formulationsdescribed herein (e.g., first and/or second modeling formulations) caninclude such curable cross-linking agents, or, alternatively or inaddition, non-curable cross-linking agents.

In some embodiments, at least some, or each of, the curable materials ineach of the first and second formulations, are (meth)acrylic materials.

Hereinthroughout, the term “(meth)acrylic” or “(meth)acrylate” anddiversions thereof encompasses both acrylic and methacrylic materials.

Acrylic and methacrylic materials encompass materials bearing one ormore acrylate, methacrylate, acrylamide and/or methacrylamide group.

Each of the curable materials can independently be a monomer, anoligomer or a polymer (which may undergo, for example, cross-linking,when cured).

Each of the curable materials can independently be a mono-functional, ormulti-functional material.

The curable materials included in the first and second formulationsdescribed herein may be defined by the properties provided by eachmaterial, when hardened. That is, the materials are defined byproperties of a material formed upon exposure to curing energy, namely,upon polymerization. These properties (e.g., Tg), are of a polymericmaterial formed upon curing any of the described curable materialsalone.

The present embodiments contemplate several types of formulations foreach of the first and second modeling formulations that are dispensed toform the layers of the object, as described herein in any of therespective embodiments. Before providing a further detailed descriptionof the modeling formulations according to some embodiments of thepresent invention, attention will be given to the advantages andpotential applications offered thereby.

The present inventors have devised a layered manufacturing AM technologythat allows building objects with improved thermo-mechanical properties,even when those properties are not possessed by any one of the modelingformulations used for fabricating the object. For example, embodimentsof the present invention provide AM structures, more preferablystructures manufactured by jetting two modeling formulations via 3Dinkjet printing technology, with high temperature resistance as well ashigh toughness. Embodiments of the present invention also allowfabricating structures with, for example, high temperature resistance aswell as low curling.

The present embodiments thus provide objects that are preferablyfabricated by AM, more preferably by 3D inkjet printing technology, andthat are characterized by HDT of at least 100° C., or at least 130° C.,or at least 140° C. The present embodiments can provide objects that arepreferably fabricated by AM, more preferably by 3D inkjet printingtechnology, and that are characterized by Izod notch impact resistanceof at least 100 J/m or at least 110 J/m or at least 120 J/m or at least130 J/m. The present embodiments can provide objects that are preferablyfabricated by AM, more preferably by 3D inkjet printing technology, andthat feature curling of less than 4 mm, or less than 3 mm.

In any of the methods and systems described herein, and objects formedthereby, at least a first modeling material formulation and a secondmodeling material formulation are utilized.

The present inventors have uncovered that using the layered structure asdescribed herein in any of the respective embodiments allows, whileselecting a first modeling material formulation and a second modelingmaterial formulation that feature certain properties, can be used toprovide at least a desired stiffness and strength of the obtainedobject. For example, increasing the content of the first formulation inthe core region can increase the strength and stiffness of thefabricated object without increasing the HDT. In another example,increasing the content of the first formulation in the core regionand/or selecting a first modeling formulation which features high Tg sum(as defined herein) can increase the damping of the core and optionallyand preferably of the fabricated object.

The present inventors have devised a layered manufacturing SFFtechnology which permits building objects with improvedthermo-mechanical properties, even when those properties are notpossessed by any one of the modeling materials used for fabricating theobject. For example, embodiments of the present invention permitfabricating structures with high temperature resistance as well as hightoughness. In the field of SFF using thermosetting materials (e.g., UVcurable materials) such properties are not possessed by any of the knownmodeling materials, since a modeling material with high temperatureresistance is relatively brittle, whereas modeling material with hightoughness has relatively low temperature resistance. Embodiments of thepresent invention also permit fabricating structures with, for example,high temperature resistance as well as low curling. Embodiments of thepresent invention also permit fabrication of structures based onelastomeric materials. For example, embodiments of the present inventionpermit the fabrication of an elastomeric structure with relatively fastreturn time as well as increased tear resistance (TR).

The modeling material can be a material contained in a particularcontainer or cartridge of a solid freeform fabrication apparatus or acombination of modeling materials deposited from different containers ofthe apparatus. The modeling materials from which the core and the shellof the present embodiments are formed, may, by themselves, have thedesired thermal and mechanical properties, according to one or more ofthe embodiments described above. This, however, need not necessarily bethe case since the Inventors of the present invention have devised atechnique for obtaining the desired properties from a combination ofmaterials. This technique will now be explained.

Suppose, for example, that it is desired to have a core having a desiredHDT. Suppose further that there is a commercially available modelingmaterial formulation, denoted formulation A, which has an HDT which ismore than the desired HDT, and another commercially available modelingmaterial formulation, denoted formulation B, which has a HDT which isless than the desired HDT. According to some embodiments of the presentinvention the core is formed from both these modeling formulations,wherein for each layer of the structure, formulations A and B areinterlaced over the core region of the layer in a voxelated manner, suchas to provide a combination which is characterized by the desired HDT.Thus, rather than mixing the materials composing a formulation inadvance using the mixture for forming the layer, the formulations A andB occupy different spatial locations which are laterally displaced fromeach other over the core region of the layer, wherein the elementarydisplacement unit of each of the materials is a single voxel. Suchcombination is referred to as digital material (DM). A representativeexample of a digital material is illustrated in FIG. 12, showingmodeling formulations A and B which are interlaced over a region of alayer in a voxelated manner.

As a representative example, consider a formulation A characterized(when hardened) by HDT of about 40° C. and a formulation B characterized(when hardened) by HDT of about 75° C. When formulations A and B aredeposited at a relative surface density ratio of A:B=3:1, namely threepixels of formulation A for each pixel of formulation B, a DMcharacterized, when hardened, by HDT of about 50° C. can be obtained.For any predetermined surface density ratio of the materials, a digitalmaterial can be formed for example, by ordered or random interlacing.Also contemplated are embodiments in which the interlacing issemi-random, for example, a repetitive pattern of sub-regions whereineach sub-region comprises random interlacing.

While the embodiments above were described with a particular emphasis toa DM for the core of the structure, it is to be understood that moredetailed reference to the core is not to be interpreted as limiting thescope of the invention in any way. Specifically, any of the core and theshell can be formed from a DM.

In some optional embodiments of the invention the thickness of theshell, as measured in the x-y plane (perpendicularly to the builddirection z) is non-uniform across the build direction. In other words,different layers of the structure may optionally have envelope regionsof different widths. For example, the thickness of the shell along adirection parallel to the x-y plane can optionally be calculated as apercentage of the diameter of the respective layer along that direction,thus making the thickness dependent on the size of the layer. In someoptional embodiments of the invention the thickness of the shell isnon-uniform across a direction which is tangential to the outer surfaceof the shell and perpendicular to the build direction. In terms of thestructure's layers, these optional embodiments correspond to an enveloperegion having a width which is non-uniform along the periphery of therespective layer.

In some optional embodiments of the present invention the shell of thestructure, or part thereof, is by itself a shelled structure, comprisingmore than envelope region. Specifically in these optional embodiments,the structure comprises an inner core, at least partially surrounded byat least one intermediate envelope region, wherein the intermediateenvelope(s) is surrounded by an outer envelope region. The thickness ofthe intermediate envelope region(s), as measured perpendicularly to thebuild direction, is optionally and preferably larger (e.g., 10 timeslarger) than the thickness of the outermost envelope region. In theseembodiments, the intermediate envelope region(s) serves as a shell ofthe structure and therefore has the properties of the shell as furtherdetailed hereinabove. The outermost envelope shell may optionally alsoserve for protecting the intermediate envelope(s) from breakage underload.

It was found by the present inventors that irregularity at the outermostinterface of the intermediate envelope region and the outermost envelopemay cause the appearance of cracks under load, such cracks spread intothe shell and possibly into the core. In some optional embodiments ofthe invention the outermost envelope provides a protective covering toprevent or reduce propagation appearance cracks at the interface betweenthe intermediate envelope and the outermost envelope regions. Theoutermost envelope can optionally also function to dissipate cracksstarting at the outermost envelop-air interface. It was also found bythe present inventors that while the capability of outermost envelope toimpede cracks appearance at the envelope-envelop interface is related tothe envelope-envelope materials moduli ratio, the capability of theoutermost envelope to withstand crack propagation from the envelope-airinterface, is related to the outermost envelope toughness. Thus,denoting the elastic modulus of the outermost envelope by ε₁, and theelastic modulus of the next-to-outermost envelope by ε₂, according tosome optional embodiments of the present invention the ratio ε₂/ε₁ isfrom about 1.3 to about 5, and the impact resistance of the outermostenvelope is at least 40 J/m or at least 50 J/m or at least 60 J/m or atleast 70 J/m, e.g., about 80 J/m or more.

Any one or combination of the above optional mechanical and thermalproperties can be achieved by a judicious selection of the propertiesand makeup of the modeling materials from which the core and shell areformed.

In some optional embodiments of the present invention the shell haslower stiffness than the core. In some experiments performed by thepresent inventors, it was found that improved thermo-mechanicalproperties can be obtained by selecting the elastic modulus ratiobetween two adjacent shells materials or between the shell (covering thecore) material and the core material, to be from about 1 to about 20. Insome embodiments, the ratio is from about 1.3 to about 5.

When it is desired to fabricate a structure with enhanced toughness, thematerial with the lowest modulus is optionally used as the outer shellmaterial and the material with the higher modulus is optionally used asthe inner shell or core material.

When it is desired to fabricate a structure with enhanced thermalresistance and reduced contribution to curling, the material with thehigher modulus is optionally used as the shell and the material with thelower modulus is optionally used as the core. Also contemplated areoptional embodiments in which an additional outermost shell is added,such that the structure has a core, and intermediate shell and anoutermost shell characterized by low curling, high temperatureresistance and high toughness, respectively.

According to some embodiments of the present invention, the firstmodeling material formulation and the second modeling materialformulation are selected so as to differ from one another, whenhardened, by at least one thermo-mechanical property. Using acombination of modeling material formulations which differ from oneanother, when hardened, by one or more thermo-mechanical propertieswithin the layered structure as described herein in any of therespective embodiments allows obtaining a final object that featuresdesirable properties in a controllable manner.

The first modeling formulation and the second modeling formulation candiffer from one another by at least one of Heat Deflection Temperature(HDT), Izod Impact resistance, Tg sum, and elastic modulus or othertensile properties, as described herein.

As used herein, HDT refers to a temperature at which the respectiveformulation or combination of formulations deforms under a predeterminedload at some certain temperature. Suitable test procedures fordetermining the HDT of a formulation or combination of formulations arethe ASTM D-648 series, particularly the ASTM D-648-06 and ASTM D-648-07methods. In various exemplary embodiments of the invention the core andshell of the structure differ in their HDT as measured by the ASTMD-648-06 method as well as their HDT as measured by the ASTM D-648-07method. In some embodiments of the present invention the core and shellof the structure differ in their HDT as measured by any method of theASTM D-648 series. In the majority of the examples herein, HDT at apressure of 0.45 MPa was used.

As used herein, the term “Izod impact resistance” refers to the loss ofenergy per unit of thickness following an impact force applied to therespective formulation or combination of formulations. Suitable testprocedures for determining the Izod impact resistance of a formulationor combination of formulations are the ASTM D-256 series, particularlythe ASTM D-256-06 series. In some embodiments of the present inventionthe core and shell of the structure differ in their Izod impactresistance value as measured by any method of the ASTM D-256-06 series.It is noted that in the standard ASTM methods there is need to machinatea notch. However, in many cases, this process cuts the shell and exposesthe core to the notch tip. Therefore, this standard method is lesspreferred for evaluating the impact resistance of a structure builtaccording to some embodiments of the present invention. Preferredprocedures for determining the impact resistance will now be described.These procedures are particularly useful when the AM includes comprisesthree-dimensional printing.

According to a first procedure, a test specimen is printed with arectangular patch made of the shelling formulation or combination offormulations. The dimensions of the patch are calculated in such waythat after the notch preparation (as required by the standard ASTMprocedure) a 0.25 mm layer of the shelling formulation or combination offormulations remains complete.

According to a second procedure, a test specimen is printed with notchinstead of cutting the notch after the specimen is printed. Theorientation of the specimen on the tray is vertical, for example, in theZ-Y plane (referred to herein as “orientation F”).

Tensile properties described herein (e.g., elastic modulus, elongationat failure, recovery) are determined in accordance with ASTMinternational standard (e.g., ASTM D638). Tensile testing characterizesan amount of tensile stress applied to the tested material as a functionof tensile strain (increase in length due to tensile stress, as apercentage of the original length) of the material.

Herein throughout, the phrase “elastic modulus” refers to Young'smodulus, as determined by response of a material to application oftensile stress.

The elastic modulus is determined as the gradient of stress as afunction of strain over ranges of stress and strain wherein stress is alinear function of strain (e.g., from a stress and strain of zero, tothe elastic proportionality limit, and optionally from zero strain to astrain which is no more than 50% of the elongation at failure).

The elongation at failure, which is also referred to herein and in theart as elongation at break, ε_(R), is determined as the maximal strain(elongation) which can occur (upon application of tensile stress equalto the ultimate tensile strength) before failure of the tested materialoccurs (e.g., as rupture or necking).

Recovery is determined by releasing the tensile stress after subjectingthe tested material as the ratio of the decrease in length to a priorstrain after a material (e.g., elastic layer) is subjected to a priorstrain which is almost equal to the elongation at failure (optionallyabout 90% of the elongation at failure, optionally about 95% of theelongation at failure, optionally about 98% of the elongation atfailure, optionally about 99% of the elongation at failure, wherein theelongation at failure can be determined using an equivalent sample).Thus, for example, a material extended to an elongation at failure whichis 200%, and which upon release of tensile stress returns to a statecharacterized by a strain of 20% relative to the original length, wouldbe characterized as having a recovery of 90% (i.e., 200%−20% divided by200%).

Herein, “Tg” refers to glass transition temperature defined as thelocation of the local maximum of the E″ curve, where E″ is the lossmodulus of the material as a function of the temperature. Broadlyspeaking, as the temperature is raised within a range of temperaturescontaining the Tg temperature, the state of a material, particularly apolymeric material, gradually changes from a glassy state into a rubberystate.

Herein, “Tg range” is a temperature range at which the E″ value is atleast half its value (e.g., can be up to its value) at the Tgtemperature as defined above.

Without wishing to be bound to any particular theory, it is assumed thatthe state of a polymeric material gradually changes from the glassystate into the rubbery within the Tg range as defined above. The lowesttemperature of the Tg range is referred to herein as Tg(low) and thehighest temperature of the Tg range is referred to herein as Tg(high).

In any of the embodiments described herein, the term “temperature higherthan Tg” means a temperature that is higher than the Tg temperature, or,more preferably a temperature that is higher than Tg(high).

Herein, “Tg sum” describes the total calculated Tg of a formulation(e.g., a modeling formulation), as calculated by summing individual Tgvalues of polymeric components of the formulation. The summation isoptionally and preferably a weight sum, wherein each Tg value ismultiplied by the relative amount (e.g., weight percentage) of therespective polymeric components of first modeling formulation. Thepolymeric components can be the respective curable components thatprovide a polymeric component featuring a Tg, or non-curable polymericcomponents added to the formulation.

According to some of any of the embodiments described herein, an HDT ofthe first modeling material formulation, when hardened, is higher thanan HDT of the second modeling material formulation, when hardened.

The HDT of the first modeling material formulation can be higher thanthe HDT of the second modeling material formulation by at least 10, orat least 20, or at least 30, or at least 40, or at least 50, or at least60, or at least 70, or at least 80, or at least 90, or at least 100° C.,including any intermediate values, and including any subranges betweenthese values.

In some embodiments an HDT of the second modeling material formulation,when hardened, is lower than 50° C., and can be, for example, any valueis the range of from about 30 to about 45° C., or from about 35 to about45° C.

In some embodiments, an HDT of the first modeling material formulation,when hardened, is higher than 50° C., and can be, for example, higherthan 60, or higher than 70, or higher than 80, preferably higher than90, or higher than 100, or higher than 110° C., or even higher.

In some embodiments an HDT of the second modeling material formulation,when hardened, is lower than 50° C. and an HDT of the first modelingmaterial formulation, when hardened, is higher than 50° C.

In some embodiments, an Izod Impact Resistance of the second modelingmaterial formulation, when hardened, is higher than an Izod ImpactResistance of the first modeling material formulation, when hardened.

The Izod Impact Resistance of the second modeling material formulation,when hardened, can higher than the Izod Impact Resistance of the firstmodeling material formulation by, for example, at least 5, or at least10, or at least 15, or at least 20, or at least 25, or at least 30, orat least 35, or at least 40, or at least 45, or at least 50, J/m, ormore, including any intermediate values and subranges between thesevalues.

In some embodiments, an Izod Impact Resistance of the second modelingmaterial formulation, when hardened, is higher than 20, or higher than25, or higher than 30, or higher than 35, or higher than 40, or higherthan 45, J/m, or even higher, and can range, for example, from 40 to 120J/m, including any intermediate value and subranges therebetween.

In some embodiments, an Izod Impact Resistance of the first modelingmaterial formulation, when hardened, ranges, for example, from 10 to 40J/m or from 10 to 30 J/m, or from 10 to 20 J/m, including anyintermediate value and subranges therebetween.

In some embodiments, the first and second modeling material formulationsare such that a ratio between elastic moduli of the first modelingmaterial formulation and the second modeling formulation, when hardened,ranges from 1 to 20, or from 1 to 10, or from 1 to 5, or from 2 to 5, orfrom 2 to 3, or from 2.5 to 3, or from 2.7 to 2.9.

According to some embodiments of the invention the first modelingmaterial formulation is characterized, when hardened, by at least oneproperty selected from the group consisting of an impact resistance fromabout 10 J/m to about 20 J/m, a heat distortion temperature at 0.45 MPafrom about 51° C. to about 150° C., a strain at break from about 2% toabout 15%, elastic modulus from about 2.3 GPa to about 3.5 GPa, andglass transition temperature from about 70° C. to about 170° C.

According to some embodiments of the invention the second modelingmaterial is characterized by at least one property selected from thegroup consisting of an impact resistance of about 45-120 J/m, a heatdistortion temperature at 0.45 MPa of about 25 to 39° C., a strain atbreak of about 40 to 100%, elastic modulus of about 0.5 to 1.5 GPa, andglass transition temperature from about 25 to 40° C.

According to some embodiments of the invention the first modelingmaterial formulation is characterized by at least one property selectedfrom the group consisting of a tensile strength from about 3 MPa toabout 5 MPa, a strain at break from about 45% to about 50%, tensile tearresistance from about 8 Kg/cm to about 12 Kg/cm, and glass transitiontemperature from about 0° C. to about 4° C.

According to some embodiments of the invention the second modelingmaterial formulation is characterized by at least one property selectedfrom the group consisting of a tensile strength from about 1 MPa toabout 2 MPa, a strain at break higher than 50%, and glass transitiontemperature from about −12° C. to about 0° C.

According to some embodiments of the invention the first modelingmaterial formulation is characterized by at least one property selectedfrom the group consisting of heat distortion temperature from about 45°C. to about 51° C. and Izod impact resistance value from about 20 J/m toabout 30 J/m.

According to some embodiments of the invention the first modelingmaterial formulation is characterized by at least one property selectedfrom the group consisting of heat distortion temperature at 0.45 MPafrom about 34° C. to about 38° C. and Izod impact resistance value fromabout 40 J/m to about 50 J/m.

In some embodiments of the present invention the first modelingformulation is characterized, when hardened, by HDT of at least 90° C.,in some embodiments of the present invention the second modelingformulation is characterized, when hardened, by Izod impact resistance(IR) value of at least 45 J/m, in some embodiments of the presentinvention the second modeling formulation is characterized, whenhardened, by HDT lower than 50° C., or lower than 45° C., and in someembodiments of the present invention a ratio between elastic moduli offirst and second modeling formulations, when hardened, is from about 2.7to about 2.9.

According to some of any of the embodiments described herein the firstmodeling formulation is characterized by, features, or selected suchthat it features, when hardened, heat deflection temperature (HDT) of atleast 90° C.

According to some of any of the embodiments described herein, the secondmodeling formulation is characterized by, features, or selected suchthat it features, when hardened, Izod impact resistance (IR) value of atleast 45 J/m.

According to some of any of the embodiments described herein the firstmodeling formulation is characterized by, features, or selected suchthat it features, when hardened, heat deflection temperature (HDT) of atleast 90° C., and the second modeling formulation is characterized by,features, or selected such that it features, when hardened, Izod impactresistance (IR) value of at least 45 J/m.

In some embodiments, the HDT of the first formulation, when hardened, isat least 100° C., or at least 110° C., or at least 120° C., or at least130° C., or at least 135° C., or at least 140° C., or higher.

In some embodiments, the Impact resistance (Izod impact resistance) ofthe second formulation is at least 47, or at least 48, or a least 49, orat least 50, or at least 51, or at least 52, or at least 53, or at least54, or at least 55, J/m, or higher.

In some of any of the embodiments described herein, the second modelingformulation is characterized by, or features, or selected so as tofeature, when hardened, by HDT lower than 50° C., or lower than 45° C.

In some embodiments the HDT of the second formulation, when hardened,ranges from 30 to 50° C., or from 35 to 50° C., or from 38 to 50° C., orfrom 40 to 50° C., or from 40 to 48° C., or from 40 to 45° C., or from30 to 45° C., or from 35 to 45° C., including any intermediate value andsubranges therebetween.

In some of any of the embodiments described herein, the first and secondformulations are characterized by, feature, or selected so as tofeature, when hardened, a ratio between elastic moduli which is lessthan 3.

In some embodiments, the ratio ranges from 2.7 to 2.9.

It was found by the present inventors that the viscoelastic propertiesof the fabricated object can be controlled by judicious selection of themodeling formulations and/or ratio between the first and second firstmodeling formulations in the various regions of the object, particularlythe core region. For example, a predetermined damping range of the core,the shell or the entire object can be obtained by selecting one or moreparameters characterizing the weight percentage of the first modelingformulation in the core, and/or by selecting one or more parameterscharacterizing the first formulation. The damping range can beexpressed, for example, using the phase lag 6 between the stress and thestrain of the core, the shell or the entire object, for a particulartemperature range.

In some of any of the embodiments described herein, the first and secondformulations are characterized by, feature, or selected so as tofeature, when hardened, the tangent of the phase lag between the stressand the strain of the respective structure (core, shell or the entireobject), is at least 0.25 at a temperature range of from about 70° C. toabout 90° C., or at least 0.20 at a temperature range of from about 90°C. to about 110° C., or at least 0.15 at a temperature range of fromabout 110° C. to about 160° C., or at least 0.15 at a temperature rangeof from about 130° C. to about 160° C.]. Thus, for example, the firstand second formulations can be selected so as to provide a desireddamping performance at the environmental temperature at which a printedobject is to be used.

The selected characteristic parameter can be the extent of cross linkingof the first formulation (expressed, for example, by the relative amountof a cross linking component (e.g., a multi-functional curablecomponent) in the first formulation). The selected characteristicparameter can alternatively or additionally be a total calculated Tg ofthe first formulation, as calculated by summing individual Tg values ofchemical components of first formulation. The summation is optionallyand preferably a weight sum, wherein each Tg value is multiplied by therelative amount (e.g., weight percentage) of the respective chemicalcomponents of first modeling formulation.

The selection of the characteristic parameter(s) can be achieved, forexample, by a look-up table having a plurality of entries, eachincluding a value indicative of the damping (e.g., the tangent of thephase 6) and a corresponding parameter or set of parameters (weightpercentage of the first modeling formulation in the core, extent ofcross linking, total calculated Tg, etc.) corresponding to the damping.The selection of the characteristic parameter(s) can alternatively oradditionally be achieved by one or more calibration curves describing avalue indicative of the damping as a function of the respectiveparameter. Representative examples of such calibration curves areprovided in the Examples section that follows (see, FIGS. 13A-E).

In various exemplary embodiments of the invention the selection isperformed by a data processor, e.g., data processor 24 or 154. Forexample, the operator can enter, via a user interface, the desireddamping or damping range, and the processor can access a memory mediumstoring a digital representation of the look-up table or calibrationcurve, and display or automatically select the parameter or set ofparameters that provide the desired damping or damping range. Theselection can optionally and preferably be based on the type of modelingformulations that are loaded to the fabrication system (e.g., the typeof modeling formulations in supply apparatus 330), so that the dataprocessor selects only the parameters that are applicable to modelingformulations already loaded into the system. Alternatively, the operatorcan also enter via the user interface, the desired modeling formulation,in which case the data processor selects only the parameters that areapplicable to the modeling formulation that was entered by the operator.

In some embodiments of the present invention the total calculated Tgvalue of the first modeling formulation is from about 100° C. to about115° C., e.g., about 107° C.

In some embodiments of the present invention the total calculated Tgvalue of the first modeling formulation is from about 120° C. to about135° C., e.g., about 127° C.

In some embodiments of the present invention the total calculated Tgvalue of the first modeling formulation is from about 140° C. to about152° C., e.g., about 146° C.

In some embodiments of the present invention the first modelingformulation includes one or more mono-functional curable material eachfeaturing a relatively high Tg (e.g., higher than 80, or higher than 85,or higher than 90, or higher than 100,° C., or higher, and at least onemulti-functional curable material, typically featuring a relatively lowTg (e.g., lower than 0° C. or lower than −10, −20, −30° C.).

In some embodiments of the present invention the ratio between the totalamount (e.g., weight percentage) of the one or more monofunctionalcurable material(s) and the multi-functional curable material is fromabout 2 to about 2.8, or from about 2.2 to about 2.6, e.g., about 2.4.

In some embodiments of the present invention the ratio between the totalamount (e.g., weight percentage) of the one or more monofunctionalcurable material(s) and the multi-functional curable material is fromabout 5 to about 6, or from about 5.2 to about 5.8, e.g., about 5.5.

In some of any of the embodiments described herein, the first modelingformulation comprises one or more curable (meth)acrylic material (e.g.,a monomer and/or an oligomer) characterized, when hardened, by Tg of atleast 50° C.

In some of any of the embodiments described herein, the first modelingformulation comprises one or more curable (meth)acrylic material (e.g.,a monomer and/or an oligomer) characterized, when hardened, by Tg of atleast 80° C.

In some of any of the embodiments described herein, the first modelingmaterial formulation comprises two or more curable materials, at leastone of the curable materials is characterized, when hardened, by Tg ofat least 80° C.

In some of any of the embodiments described herein, the first modelingformulation comprises one or more curable (meth)acrylic material (e.g.,a monomer and/or an oligomer) characterized, when hardened, by a highTg, e.g., a Tg of at least 100° C., or at least 110, or at least 120, orat least 130, or at least 140, or at least 150° C.

In some of any of the embodiments described herein, the first modelingmaterial formulation comprises two or more curable (meth)acrylicmaterials, at least one of the materials is characterized, whenhardened, by a high Tg as described herein.

In some of these embodiments, the curable material featuring such a highTg is a monomer, and in some embodiments it is a bifunctional monomer.

In some embodiments, the curable material featuring such a high Tg is amethacrylate monomer.

A non-limiting, exemplary methacrylic monomer which is characterized,when hardened, by Tg higher than 150° C., and/or by a curing rate asdescribed herein, is SR 843 (Tricyclodecanedimethanol dimethacrylate(TCDDMDMA)). An additional exemplary such material is sold under thebrand name SR-423D. See, for example Table 1.

In some of any of the embodiments described herein, the first modelingmaterial formulation comprises: a curable (meth)acrylic material (e.g.,a monomer and/or an oligomer) characterized, when hardened, by Tg of atleast 50° C.; a curable (meth)acrylic material (e.g., a monomer and/oran oligomer) characterized, when hardened, by Tg of at least 80° C.; anda curable (meth)acrylic material (e.g., a monomer and/or an oligomer)characterized, when hardened, by a high Tg, e.g., a Tg of at least 100°C., or at least 110, or at least 120, or at least 130, or at least 140,or at least 150° C.

In some of any of the embodiments described herein, the first modelingmaterial formulation comprises a curable acrylic monomer characterized,when hardened, by Tg of at least 85° C.;

a curable methacrylic monomer characterized, when hardened, by Tg of atleast 150° C.; and

a curable (meth)acrylic oligomer, characterized, when hardened, by Tg ofat least 50° C.

In some of any of the embodiments described herein, the first modelingmaterial formulation comprises: at least one curable (meth)acrylicmonomer; at least one curable (meth)acrylic oligomer; and optionally, atleast one curable (meth)acrylic monomer characterized, when hardened, byTg lower than 0° C., or lower.

In some of any of the embodiments described herein, the first modelingmaterial formulation comprises: at least one curable (meth)acrylicmonomer characterized, when hardened, by Tg of at least 85° C.; at leastone curable (meth)acrylic monomer characterized, when hardened, by Tg ofat least 150° C.; at least one curable (meth)acrylic oligomer,characterized, when hardened, by Tg of at least 50° C.; and optionally,at least one curable (meth)acrylic monomer characterized, when hardened,by Tg lower than 0° C.

In some of the embodiments in which the first modeling materialformulation comprises two or more (e.g., three) types of curablematerials, a concentration of each of the curable materials in theformulation independently ranges from 10 to 60% by weight of the totalweight of the first formulation.

In some embodiments, acrylic monomers characterized, when hardened, byTg higher than 85° C. include monofunctional, difunctional, othermultifunctional acrylate monomers, and any mixture thereof. In someembodiments, the Tg of the acrylate monomers ranges from 86 to about300° C.

The acrylate monomers featuring such Tg can be, for example, commonlyused monofunctional acrylate monomers such as ACMO and IBOA;multifunctional acrylate monomers such as, for example, Tris(2-hydroxyethyl) isocyanurate triacrylate (THEICTA), commerciallyavailable under the name SE368; short-chain alkylene glycol-containing(ethoxylated) difunctional and trifunctional acrylate monomers such as,for example, DPGDA (commercially available under the name SR508),ethoxylated 3 trimethylolpropane triacrylate (TMP3EOTA), commerciallyavailable under the name SR454, and long-chain or high-carbon ringmultifunctional acrylate monomers such as, for example,Tricyclodecanedimethanol diacrylate (TCDDMDA), commercially availableunder the name SR833S.

Exemplary acrylic monomers characterized, when hardened, by Tg higherthan 85° C. include, but are not limited to, those presented in Table 1hereinbelow. Any other acrylic monomer featuring the indicated Tg iscontemplated. Those skilled in the art would readily recognizeadditional acrylate monomers featuring Tg higher than 85° C.

The acrylic monomer featuring the indicated Tg, when hardened, can be amixture of two or more such monomers.

In some embodiments, the (meth)acrylic oligomer is characterized, whenhardened, by Tg of at least 50° C., is or comprises an acrylic oligomer,or, alternatively a mixture of two or more acrylic monomers or of one ormore acrylic monomers and one or more methacrylic monomers.

Exemplary such oligomers include, but are not limited to, polyesterurethane acrylates, epoxy acrylates, modified (e.g., amine modified)epoxy acrylates and the like. Any other acrylic oligomer featuring theindicated Tg is contemplated. Those skilled in the art would readilyrecognize additional acrylate oligomers featuring Tg higher than 50° C.

In some embodiments, the first modeling formulation may further compriseat least one curable (meth)acrylic monomer which provides, whenhardened, a flexible material, characterized by Tg lower than 0° C., orlower than −10° C., or lower than −20° C.

In some embodiments, the (meth)acrylic monomer characterized, whenhardened, by Tg lower than −10 or −20° C., is or comprises an acrylicmonomer, or, alternatively a mixture of two or more acrylic monomers orof one or more acrylic monomers and one or more methacrylic monomers.

Acrylic and methacrylic monomers featuring such low Tg include, forexample, ethoxylated monofunctional, or preferably multifunctional(e.g., difunctional or trifunctional), as described herein in any of therespective embodiments.

Exemplary such flexible acrylic monomers are presented in Table 1 below.Any other flexible acrylic (or methacrylic) monomers are contemplated.Those skilled in the art would readily recognize additional acrylatemonomers featuring low Tg as indicated.

Herein, an “ethoxylated” material describes an acrylic or methacryliccompound which comprises one or more alkylene glycol groups, or,preferably, one or more alkylene glycol chains, as defined herein.Ethoxylated (meth)acrylate materials can be monofunctional, or,preferably, multifunctional, namely, difunctional, trifunctional,tetrafunctional, etc.

In multifunctional materials, typically, each of the (meth)acrylategroups are linked to an alkylene glycol group or chain, and the alkyleneglycol groups or chains are linked to one another through a branchingunit, such as, for example, a branched alkyl, cycloalkyl, aryl (e.g.,bisphenol A), etc.

In some embodiments, the ethoxylated material comprises at least 5ethoxylated groups, that is, at least 5 alkylene glycol moieties orgroups. Some or all of the alkylene glycol groups can be linked to oneanother to form an alkylene glycol chain. For example, an ethoxylatedmaterial that comprises 30 ethoxylated groups can comprise a chain of 30alkylene glycol groups linked to one another, two chains, each, forexample, of 15 alkylene glycol moieties linked to one another, the twochains linked to one another via a branching moiety, or three chains,each, for example, of 10 alkylene glycol groups linked to one another,the three chains linked to one another via a branching moiety. Shorterand longer chains are also contemplated.

In some embodiments, the ethoxylated material comprises at least 8, orat least 10, or at least 12, or at least 15, or at least 18, or at least20, or at least 25, or at least 30 ethoxylated (alkylene glycol) groups.The ethoxylated material can comprise one, two or more poly(alkyleneglycol) chains, of any length, as long as the total number of thealkylene glycol groups is as indicated.

In some embodiments, a concentration of the flexible (meth)acrylicmonomer, if present, in the first formulation, ranges from 4 to 30, orfrom 4 to 25, or from 4 to 20, or from 4 to 15, or from 4.5 to 13.5weight percents, of the total weight of the formulation, including anyintermediate values and subranges therebetween.

In some embodiments, the flexible monomer is a multi-functionalethoxylated monomer as described herein, in which each of the(meth)acrylate groups are linked to an alkylene glycol group or chain,and the alkylene glycol groups or chains are linked to one anotherthrough a branching unit, such as, for example, a branched alkyl,cycloalkyl, aryl (e.g., bisphenol A), etc., as described in furtherdetail hereinunder.

In some of any of the embodiments described herein, the first modelingformulation further comprises an additional curable (meth)acrylicmonomer which provides, when hardened, a flexible material,characterized by Tg lower than 0° C., or lower than −10° C., or lowerthan −20° C.

In some embodiments, the additional flexible monomer is a di-functionalmonomer which comprises an alkylene glycol chain (a poly(alkyleneglycol, as defined herein) that terminates at both ends by an acrylateor methacrylate group.

In some embodiments, the poly(alkylene glycol) chain features at least5, preferably at least 10, e.g., from 10 to 15, alkylene glycol groups.

In some embodiments, the concentration of the additional flexiblemonomer as described herein ranges from 5 to 20, or from 5 to 15, weightpercents, of the total weight of the formulation, including anyintermediate values and subranges therebetween.

In some of any of the embodiments described herein, the firstformulation comprises, as a flexible monomer having a low Tg asindicated herein only the material described herein as “an additionalflexible monomer”.

In some embodiments, the first formulation is used in combination withany second formulation that features, when hardened, Impact resistancevalue and/or HDT, as defined herein.

In some embodiments, the second modeling formulation comprises one ormore curable materials, at least one of the curable materials being aflexible curable (meth)acrylic material (e.g., a monomer or anoligomer), preferably an acrylic monomer, characterized, when hardened,by Tg lower than 0° C., or lower than −10° C. or lower than −20° C., asdescribed herein.

In some embodiments, the second modeling material formulation comprisesat least one curable (meth)acrylic material, preferably an acrylicmonomer, characterized, when hardened, by Tg of at least 50° C., or atleast 60° C., or at least 70° C.

In some embodiments, the second modeling material formulation comprisesat least one curable (meth)acrylic material, preferably an acrylicoligomer, characterized, when hardened, by Tg of at least 10° C.

In some embodiments, the second modeling material formulation comprisesat least two curable materials, at least one of the curable materialsbeing a (meth)acrylic monomer characterized, when hardened, by Tg lowerthan −20° C.

In some of these embodiments, the second modeling material formulationfurther comprises at least one curable (meth)acrylic monomercharacterized, when hardened, by Tg of at least 70° C.

In some of these embodiments, the second modeling material formulationfurther comprises at least one curable (meth)acrylic oligomercharacterized, when hardened, by Tg of at least 10° C. In someembodiments, the second modeling formulation comprises:

at least one curable (meth)acrylic, preferably acrylic, monomercharacterized, when hardened, by Tg of at least 50, or at least 60, orat least 70° C.;

at least one curable (meth)acrylic, preferably acrylic, oligomercharacterized, when hardened, by Tg of at least 10° C.; and

at least one flexible curable (meth)acrylic, preferably acrylic, monomercharacterized, when hardened, by Tg lower than 0, or lower than −10 orlower than −20° C., as described herein.

In some of any of the embodiments described herein, the secondformulation comprises at least one flexible curable material, asdescribed herein; at least one curable (meth)acrylic, preferablyacrylic, monomer characterized, when hardened, by Tg of at least 50, 60or 70° C.; and at least one curable (meth)acrylic, preferably acrylic,oligomer characterized, when hardened, by Tg of at least 10° C.

In some embodiments, the curable (meth)acrylic, preferably acrylic,monomer characterized, when hardened, by Tg of at least 50° C., ischaracterized by Tg of at least 85° C., when hardened, and includemonofunctional and multifunctional monomers, and any mixture of suchmonomers, as described herein.

Curable (meth)acrylic oligomers characterized, when hardened, by Tg ofat least 10° C., include monofunctional, and preferably multifunctionaloligomers such as, but not limited to, polyester urethane acrylates,epoxy acrylates, modified epoxy acrylates, etc. Those skilled in the artwould readily recognize oligomers featuring the indicated Tg.

In some embodiments, when the second formulation comprises two or moretypes of curable materials, the concentration of each curable materialindependently ranges from 10 to 50% by weight of the total weight of thesecond modeling formulation.

Modeling material formulations featuring thermo-mechanical features asdescribed herein include commercially available formulations andformulations designed so as to feature the indicated thermo-mechanicalproperties.

In some embodiments of the present invention, the first and secondmodeling material formulations are selected such that when used forforming a layered structure as described herein in any of the respectiveembodiments, the core has an HDT which is below 60° C., 50° C. or below40° C. or below 30° C. and the shell has an HDT which is above 60° C. orabove 50° C. or above 40° C. or above 30° C., as measured by an ASTMstandard method, as further detailed herein. In such embodiments, anobject with relatively low curling and high temperature resistance canbe obtained, the low HDT core is responsible for the low curlingtendency and the high HDT of the shell contributes to high temperatureresistance of the fabricated object.

In some embodiments, the core and shell of the fabricated structurediffer in their heat distortion temperature (HDT) and/or Izod impactresistance (IR). For example, the IR characterizing the core can belower than the IR characterizing the shell, and the HDT characterizingthe core is can be higher than the HDT characterizing the shell. In thisembodiment, the high HDT core induces high temperature resistance andthe high IR of the shell imparts toughness to the entire object producedwith such core-shell structure and materials. Optionally and preferablyboth relations are fulfilled for the same structure, namely the IR valueis lower for the core region than for the shell, but the HDT is higherfor the core region than for the shell.

In some embodiments of the present invention the core is made of amaterial characterized by a HDT at pressure of 0.45 MPa which is fromabout 40° C. to about 50° C. A representative example of a modelingmaterial having such thermal properties is a modeling material marketedby Objet Geometries under the trade name VeroGray™. In some embodimentsof the present invention the shell is made of a material characterizedby an IR value of from about 40 J/m to about 50 J/m, e.g., about 40 J/m.A representative example of a modeling material having such thermalproperties is a modeling material marketed by Objet Geometries under thetrade name DurusWhite™.

In some embodiments of the present invention both the core and the shellare or include formulations or combination of formulations that providerubber-like polymeric materials (e.g., elastomeric materials).

As used herein, the term “rubber-like material” refers to a materialwhich is characterized by Tg which is significantly lower than roomtemperature. For example Tg of about 10° C. or less.

When the core and shell are made of a rubber-like material, the corematerial may have a lower elongation at break value ε_(R) than the shellmaterial, e.g. ε_(R)>1%. Preferably, there is a difference of at least30% or at least at least 60% or at least 90% or at least 120% betweenthe ε_(R) of the core and the ε_(R) of the shell. For example, when thecore has an ε_(R) value of 50% the shell has an ε_(R) value which is atleast 30% larger, namely an ε_(R) value of 80% or more. Typically, butnot necessarily, the ε_(R) value of the shell is at least 100%.

In some embodiments of the present invention the core is made of amaterial characterized by TR of from about 2 Kg/cm to about 12 Kg/cm,e.g., about 4 Kg/cm or about 10 Kg/cm, and an ε_(R) value of from about45% to about 50%. In some embodiments, the material also possesses oneor more of the following properties:

tensile strength of from about 1 MPa to about 5 MPa, e.g., about 2 MPaor about 4.36 MPa, and glass transition temperature from about −12° C.to about 4° C., e.g., about −10.7° C. or about 2.6° C. Representativeexamples of modeling materials having such thermal properties aremodeling materials marketed by Objet Geometries under the trade namesTangoBlack™ and TangoGray™.

In some embodiments of the present invention the shell is made of amaterial characterized by TR from about 2 Kg/cm to about 4 Kg/cm, e.g.,about 3 Kg/cm, and an ε_(R) value from about 200% to about 236%. In someembodiments, the material also possesses one or more of the followingproperties: tensile strength of from about 1 MPa to about 2 MPa, andglass transition temperature from about −12° C. to about −8° C.Representative examples of modeling materials having such thermalproperties are modeling materials marketed by Objet Geometries under thetrade names TangoBlack Plus™ and Tango Plus™.

According to some embodiments of the invention for at least one pair ofregions in the layer, a heat deflection temperature (HDT) characterizingan inner region of the pair is above 50° C., and an HDT characterizingan outer region of the pair is below 50° C.

According to some embodiments of the invention for at least one pair ofregions in the layer, an outer region of the pair has a lower elasticmodulus than an inner region of the pair.

According to some embodiments of the invention for at least one pair ofregions in the layer, an outer region of the pair has a higher elasticmodulus than an inner region of the pair.

According to some embodiments of the invention for at least one pair ofregions in the layer, an outer region of the pair has a higher elasticmodulus than an inner region of the pair.

According to some embodiments of the invention for any pair of regionsin the layer, an outer region of the pair has a lower elastic modulusthan an inner region of the pair.

According to some embodiments of the invention a heat deflectiontemperature (HDT) characterizing the core region is below about 50° C.and an HDT characterizing at least one of the envelope regions is aboveabout 50° C. According to some embodiments of the invention for at leastone pair of envelope regions, an HDT characterizing an inner enveloperegion of the pair is above 50° C., and an HDT characterizing an outerenvelope region of the pair is below 50° C. According to someembodiments of the invention for at least one pair of envelope regions,an HDT characterizing an inner envelope region of the pair is above 50°C., and an HDT characterizing an outer envelope region of the pair isbelow 50° C. According to some embodiments of the invention for at leastone pair of regions in the layer, a characteristic HDT is higher for anouter region of the pair than for an inner region of the pair.

According to some embodiments of the invention each of the core andenvelope regions being characterized by an elongation-at-break value(ε_(R)), when hardened, wherein the characteristic ε_(R) is higher forany of the envelope regions than for the core region. According to someembodiments of the invention for any pair of regions in the layer, thecharacteristic ε_(R) is higher for an outer region of the pair than foran inner region of the pair. According to some embodiments of theinvention for at least one pair of regions in the layer, thecharacteristic ε_(R) of an outer region of the pair is higher by atleast 30% than the characteristic ε_(R) of an inner region of the pair.According to some embodiments of the invention for at least one pair ofregions in the layer, a characteristic ε_(R) of an outer region of thepair is at least 30%, and a characteristic ε_(R) of an inner region ofthe pair is from about 2% to about 15%.

According to some embodiments of the invention the first modelingmaterial and the second modeling material are characterized by a glasstransition temperature (Tg) which is below 10° C. According to someembodiments of the invention for at least one pair of regions in thelayer, the characteristic ε_(R) of an outer region of the pair is atleast 200%, and the characteristic ε_(R) of an inner region of the pairis from about 1% to about 100%. According to some embodiments of theinvention a characteristic tensile tear resistance (TR) of the coreregion is lower than a characteristic TR of at least one of the enveloperegions.

According to some embodiments of the invention each of the regions ischaracterized by an Izod impact resistance (IR) value and an HDT, whenhardened, wherein for at least one pair of regions in the layer, aninner region of the pair is characterized a lower IR value and higherHDT value relative to an outer region of the pair. According to someembodiments of the invention the inner region is characterized by an IRvalue of about 20 J/m. According to some embodiments of the inventionthe outer region is characterized by IR value of at least 40 J/m.According to some embodiments of the invention the inner region ischaracterized by HDT of at least 60° C. According to some embodiments ofthe invention the inner region is characterized by HDT of at most 50° C.

According to some embodiments of the invention for at least one pair ofregions in the layer, an inner region of the pair is characterized ahigher IR value and higher HDT value relative to an outer region of thepair.

According to some embodiments of the invention for at least one pair ofregions in the layer, an inner region of the pair is characterized ahigher IR value and lower HDT value relative to an outer region of thepair.

In some of any of the embodiments described herein, each of the firstand second modeling material formulations independently comprises aphotoinitiator, for initiating the polymerization or cross-linking(curing) upon exposure to curing energy (e.g., radiation).

In some embodiments, the photoinitiator is a free-radical initiator.

A free radical photoinitiator may be any compound that produces a freeradical on exposure to radiation such as ultraviolet or visibleradiation and thereby initiates a polymerization reaction. Non-limitingexamples of suitable photoinitiators include benzophenones (aromaticketones) such as benzophenone, methyl benzophenone, Michler's ketone andxanthones; acylphosphine oxide type photo-initiators such as2,4,6-trimethylbenzolydiphenyl phosphine oxide (TMPO),2,4,6-trimethylbenzoylethoxyphenyl phosphine oxide (TEPO), monoacylphosphine oxides (MAPO's) and bisacylphosphine oxides (BAPO's); benzoinsand benzoin alkyl ethers such as benzoin, benzoin methyl ether andbenzoin isopropyl ether and the like. Examples of photoinitiators arealpha-amino ketone, alpha-hydroxy ketone, monoacyl phosphine oxides(MAPO's) and bisacylphosphine oxide (BAPO's).

A free-radical photo-initiator may be used alone or in combination witha co-initiator. Co-initiators are used with initiators that need asecond molecule to produce a radical that is active in the UV-systems.Benzophenone is an example of a photoinitiator that requires a secondmolecule, such as an amine, to produce a curable radical. Afterabsorbing radiation, benzophenone reacts with a ternary amine byhydrogen abstraction, to generate an alpha-amino radical which initiatespolymerization of acrylates. Non-limiting example of a class ofco-initiators are alkanolamines such as triethylamine,methyldiethanolamine and triethanolamine.

In some embodiments, a concentration of the initiator in the firstand/or the second modeling material formulation independently rangesfrom 0.5 to 5%, or from 1 to 5%, or from 2 to 5%, by weight of the totalweight of the respective formulation.

In some of any of the embodiments described herein, the first and/orsecond modeling material formulation independently further comprises oneor more additional materials, which are referred to herein also asnon-reactive materials (non-curable materials).

Such agents include, for example, surface active agents (surfactants),inhibitors, antioxidants, fillers, pigments, dyes, and/or dispersants.

Surface-active agents may be used to reduce the surface tension of theformulation to the value required for jetting or for printing process,which is typically around 30 dyne/cm. Such agents include siliconematerials, for example, organic polysiloxanes such as PDMS andderivatives therefore, such as those commercially available as BYK typesurfactants.

Suitable dispersants (dispersing agents) can also be silicone materials,for example, organic polysiloxanes such as PDMS and derivativestherefore, such as those commercially available as BYK type surfactants.

Suitable stabilizers (stabilizing agents) include, for example, thermalstabilizers, which stabilize the formulation at high temperatures.

The term “filler” describes an inert material that modifies theproperties of a polymeric material and/or adjusts a quality of the endproducts. The filler may be an inorganic particle, for example calciumcarbonate, silica, and clay.

Fillers may be added to the modeling formulation in order to reduceshrinkage during polymerization or during cooling, for example, toreduce the coefficient of thermal expansion, increase strength, increasethermal stability, reduce cost and/or adopt rheological properties.Nanoparticles fillers are typically useful in applications requiring lowviscosity such as ink-jet applications.

In some embodiments, a concentration of each of a surfactant and/or adispersant and/or a stabilizer and/or a filler, if present, ranges from0.01 to 2%, or from 0.01 to 1%, by weight, of the total weight of therespective formulation. Dispersants are typically used at aconcentration that ranges from 0.01 to 0.1%, or from 0.01 to 0.05%, byweight, of the total weight of the respective formulation.

In some embodiments, the first and/or second formulation furthercomprises an inhibitor. The inhibitor is included for preventing orreducing curing before exposure to curing energy. Suitable inhibitorsinclude, for example, those commercially available as the Genorad type,or as MEHQ. Any other suitable inhibitors are contemplated.

The pigments can be organic and/or inorganic and/or metallic pigments,and in some embodiments the pigments are nanoscale pigments, whichinclude nanoparticles.

Exemplary inorganic pigments include nanoparticles of titanium oxide,and/or of zinc oxide and/or of silica. Exemplary organic pigmentsinclude nanosized carbon black.

In some embodiments, the pigment's concentration ranges from 0.1 to 2%by weight, or from 0.1 to 1.5%, by weight, of the total weight of therespective formulation.

In some embodiments, the first formulation comprises a pigment.

In some embodiments, combinations of white pigments and dyes are used toprepare colored cured materials.

The dye may be any of a broad class of solvent soluble dyes. Somenon-limiting examples are azo dyes which are yellow, orange, brown andred; anthraquinone and triarylmethane dyes which are green and blue; andazine dye which is black.

Any of the first and/or second formulations described herein, in any ofthe respective embodiments and any combination thereof, can be providedwithin a kit, in which the first and second formulations areindividually packaged.

In some embodiments, all the components of each formulation are packagedtogether. In some of these embodiments, the formulations are packaged ina packaging material which protects the formulations from exposure tolight or any other radiation and/or comprise an inhibitor.

In some embodiments, the initiator is packaged separately from othercomponents of each formulation, and the kit comprises instructions toadd the initiator to the respective formulation.

The present inventors have devised a technique that further reduces thecurling effect. In this technique, a structure, referred to herein as “apedestal” is dispensed directly on the tray, and the layers that make upthe object are thereafter dispensed on the pedestal. This embodiment isillustrated in FIGS. 10A-B.

FIG. 10A shows a side view of a pedestal 202 on tray 360 wherein thelayers of an object 200 are dispensed on pedestal 202. Object 200 cancomprise, or be, a shelled structure (e.g., structure 60), made of thefirst and second modeling formulations as further detailed hereinabove.Alternatively, object 200 can be a non-shelled structure, or a shelledstructure (e.g., structure 60), made of other modeling formulation, suchas a commercially available modeling formulation.

Pedestal 202 optionally and preferably serves to ease the removal ofobject from the printing tray and thus may help prevent deformation bymanual or mechanical damage. Pedestal 202 can also improve the object'saccuracy in the Z direction (height), and/or may improve an object'saccuracy in the X-Y directions.

Pedestal 202 preferably comprises a support formulation that includes asupport material. Preferably the support formulation is soluble inliquid, e.g., in water. In various exemplary embodiments of theinvention pedestal 202 comprises a combination of support formulationand modeling formulation (e.g., any of the first and second modelingformulations described herein). Preferably, the modeling formulationwithin pedestal 202 is of low Izod impact resistance, for example, lessthan 40 J/m. The advantage of this embodiment is that it reduces thetendency of the pedestal to lift from the tray.

Inaccuracies in Z may occur at the lowest layers of the printed object.This may be because the top surface of the tray at Z start level (the Zlevel of the tray when printing starts) may not be exactly at a heightwhich enables the leveling device to reach and thus level the firstlayers deposited in the printing process, when the leveling device maybe at its lowest point (e.g., because of inaccuracy in adjustmentsand/or incomplete flatness and horizon of the tray). As a result, thelower layers of object 200 may not be leveled by the leveling device andtherefore their thickness may be greater than the designed layerthickness, therefore increasing the height of object 200 as printed incontrast to the object as designed. The use of pedestal 202 under thelowest point of the object solves this problem by specifying that theheight at which the printing of the actual object starts may be theheight at which the pedestal itself may be significantly leveled by theleveling device.

In various exemplary embodiments of the invention pedestal 202 has acore-shell structure, in which the shell comprises spaced pillars ofmodeling formulation with support formulation in-between, and the corecomprises only soluble (e.g., water soluble) support formulation, and isdevoid of any non-soluble modeling formulation. These embodiments areillustrated in FIG. 10B which is a top view illustration of a typicallayer of pedestal 202, having a pedestal core (shown as a core region208 in FIG. 10B) and pedestal shell (shown as an envelope region 210 inFIG. 10B). The support formulation is shown at 204 (patterned filling)and the modeling formulation pillars are shown at 206 (white filling).

The advantage of forming a pedestal with core-shell structure as definedabove is that it solves the problems of Z inaccuracies and curling whileminimizing the use of modeling formulation, which is typically moreexpensive, and tends to leave visible remnants at the bottom of object200.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

Herein throughout, the term “object” describes a final product of theadditive manufacturing. This term refers to the product obtained by amethod as described herein, after removal of the support material, ifsuch has been used as part of the building material. The “object”therefore essentially consists (at least 95 weight percents) of ahardened (e.g., cured) modeling material.

The term “object” as used herein throughout refers to a whole object ora part thereof.

Herein throughout, the phrases “building material formulation”, “uncuredbuilding material”, “uncured building material formulation”, “buildingmaterial” and other variations therefore, collectively describe thematerials that are dispensed to sequentially form the layers, asdescribed herein. This phrase encompasses uncured materials dispensed soas to form the object, namely, one or more uncured modeling materialformulation(s), and uncured materials dispensed so as to form thesupport, namely uncured support material formulations.

Herein throughout, the phrase “cured modeling material” or “hardenedmodeling material” describes the part of the building material thatforms the object, as defined herein, upon exposing the dispensedbuilding material to curing, and, optionally, if a support material hasbeen dispensed, also upon removal of the cured support material, asdescribed herein. The cured modeling material can be a single curedmaterial or a mixture of two or more cured materials, depending on themodeling material formulations used in the method, as described herein.

The phrase “cured modeling material” or “cured modeling materialformulation” can be regarded as a cured building material wherein thebuilding material consists only of a modeling material formulation (andnot of a support material formulation). That is, this phrase refers tothe portion of the building material, which is used to provide the finalobject.

Herein throughout, the phrase “modeling material formulation”, which isalso referred to herein interchangeably as “modeling formulation”,“model formulation” “model material formulation” or simply as“formulation”, describes a part or all of the building material which isdispensed so as to form the object, as described herein. The modelingmaterial formulation is an uncured modeling formulation (unlessspecifically indicated otherwise), which, upon exposure to curingenergy, forms the object or a part thereof.

In some embodiments of the present invention, a modeling materialformulation is formulated for use in three-dimensional inkjet printingand is able to form a three-dimensional object on its own, i.e., withouthaving to be mixed or combined with any other substance.

The phrase “digital materials”, as used herein and in the art, describesa combination of two or more materials on a microscopic scale or voxellevel such that the printed zones of a specific material are at thelevel of few voxels, or at a level of a voxel block. Such digitalmaterials may exhibit new properties that are affected by the selectionof types of materials and/or the ratio and relative spatial distributionof two or more materials.

In exemplary digital materials, the modeling material of each voxel orvoxel block, obtained upon curing, is independent of the modelingmaterial of a neighboring voxel or voxel block, obtained upon curing,such that each voxel or voxel block may result in a different modelmaterial and the new properties of the whole part are a result of aspatial combination, on the voxel level, of several different modelmaterials.

Herein throughout, whenever the expression “at the voxel level” is usedin the context of a different material and/or properties, it is meant toinclude differences between voxel blocks, as well as differences betweenvoxels or groups of few voxels. In preferred embodiments, the propertiesof the whole part are a result of a spatial combination, on the voxelblock level, of several different model materials.

The term “branching unit” as used herein describes a multi-radical,preferably aliphatic or alicyclic moiety, and optionally an aryl orheteroaryl moiety. By “multi-radical” it is meant that the branchingunit has two or more attachment points such that it links between two ormore atoms and/or groups or moieties.

That is, the branching unit is a chemical moiety that, when attached toa single position, group or atom of a substance, creates two or morefunctional groups that are linked to this single position, group oratom, and thus “branches” a single functionality into two or morefunctionalities.

In some embodiments, the branching unit is derived from a chemicalmoiety that has two, three or more functional groups. In someembodiments, the branching unit is a branched alkyl or a branchedlinking moiety as described herein.

As used herein the term “about” refers to ±10% or ±5%.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of” means “including and limited to”.

The term “consisting essentially of” means that the composition, methodor structure may include additional ingredients, steps and/or parts, butonly if the additional ingredients, steps and/or parts do not materiallyalter the basic and novel characteristics of the claimed composition,method or structure.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise. For example,the term “a compound” or “at least one compound” may include a pluralityof compounds, including mixtures thereof.

Throughout this application, various embodiments of this invention maybe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numberswithin that range, for example, 1, 2, 3, 4, 5, and 6. This appliesregardless of the breadth of the range.

Whenever a numerical range is indicated herein, it is meant to includeany cited numeral (fractional or integral) within the indicated range.The phrases “ranging/ranges between” a first indicate number and asecond indicate number and “ranging/ranges from” a first indicate number“to” a second indicate number are used herein interchangeably and aremeant to include the first and second indicated numbers and all thefractional and integral numerals therebetween.

As used herein, the term “amine” describes both a —NR′R″ group and a—NR′— group, wherein R′ and R″ are each independently hydrogen, alkyl,cycloalkyl, aryl, as these terms are defined hereinbelow.

The amine group can therefore be a primary amine, where both R′ and R″are hydrogen, a secondary amine, where R′ is hydrogen and R″ is alkyl,cycloalkyl or aryl, or a tertiary amine, where each of R′ and R″ isindependently alkyl, cycloalkyl or aryl.

Alternatively, R′ and R″ can each independently be hydroxyalkyl,trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl,heteroalicyclic, amine, halide, sulfonate, sulfoxide, phosphonate,hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy, thioaryloxy, cyano,nitro, azo, sulfonamide, carbonyl, C-carboxylate, O-carboxylate,N-thiocarbamate, O-thiocarbamate, urea, thiourea, N-carbamate,O-carbamate, C-amide, N-amide, guanyl, guanidine and hydrazine.

The term “amine” is used herein to describe a —NR′R″ group in caseswhere the amine is an end group, as defined hereinunder, and is usedherein to describe a —NR′— group in cases where the amine is a linkinggroup or is or part of a linking moiety.

The term “alkyl” describes a saturated aliphatic hydrocarbon includingstraight chain and branched chain groups. Preferably, the alkyl grouphas 1 to 30, or 1 to 20 carbon atoms. Whenever a numerical range; e.g.,“1-20”, is stated herein, it implies that the group, in this case thealkyl group, may contain 1 carbon atom, 2 carbon atoms, 3 carbon atoms,etc., up to and including 20 carbon atoms. The alkyl group may besubstituted or unsubstituted. Substituted alkyl may have one or moresubstituents, whereby each substituent group can independently be, forexample, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl,heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide,phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy,thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate,O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea,N-carbamate, O-carbamate, C-amide, N-amide, guanyl, guanidine andhydrazine.

The alkyl group can be an end group, as this phrase is definedhereinabove, wherein it is attached to a single adjacent atom, or alinking group, as this phrase is defined hereinabove, which connects twoor more moieties via at least two carbons in its chain. When the alkylis a linking group, it is also referred to herein as “alkylene” or“alkylene chain”.

Herein, a C(1-4) alkyl, substituted by a hydrophilic group, as definedherein, is included under the phrase “hydrophilic group” herein.

Alkene and Alkyne, as used herein, are an alkyl, as defined herein,which contains one or more double bond or triple bond, respectively.

The term “cycloalkyl” describes an all-carbon monocyclic ring or fusedrings (i.e., rings which share an adjacent pair of carbon atoms) groupwhere one or more of the rings does not have a completely conjugatedpi-electron system. Examples include, without limitation, cyclohexane,adamantine, norbornyl, isobornyl, and the like. The cycloalkyl group maybe substituted or unsubstituted. Substituted cycloalkyl may have one ormore substituents, whereby each substituent group can independently be,for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl,aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide,phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy,thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate,O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea,N-carbamate, O-carbamate, C-amide, N-amide, guanyl, guanidine andhydrazine. The cycloalkyl group can be an end group, as this phrase isdefined hereinabove, wherein it is attached to a single adjacent atom,or a linking group, as this phrase is defined hereinabove, connectingtwo or more moieties at two or more positions thereof.

Cycloalkyls of 1-6 carbon atoms, substituted by two or more hydrophilicgroups, as defined herein, is included under the phrase “hydrophilicgroup” herein.

The term “heteroalicyclic” describes a monocyclic or fused ring grouphaving in the ring(s) one or more atoms such as nitrogen, oxygen andsulfur. The rings may also have one or more double bonds. However, therings do not have a completely conjugated pi-electron system.Representative examples are piperidine, piperazine, tetrahydrofurane,tetrahydropyrane, morpholino, oxalidine, and the like.

The heteroalicyclic may be substituted or unsubstituted. Substitutedheteroalicyclic may have one or more substituents, whereby eachsubstituent group can independently be, for example, hydroxyalkyl,trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl,heteroalicyclic, amine, halide, sulfonate, sulfoxide, phosphonate,hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy, thioaryloxy, cyano,nitro, azo, sulfonamide, C-carboxylate, O-carboxylate, N-thiocarbamate,O-thiocarbamate, urea, thiourea, O-carbamate, N-carbamate, C-amide,N-amide, guanyl, guanidine and hydrazine. The heteroalicyclic group canbe an end group, as this phrase is defined hereinabove, where it isattached to a single adjacent atom, or a linking group, as this phraseis defined hereinabove, connecting two or more moieties at two or morepositions thereof.

A heteroalicyclic group which includes one or more of electron-donatingatoms such as nitrogen and oxygen, and in which a numeral ratio ofcarbon atoms to heteroatoms is 5:1 or lower, is included under thephrase “hydrophilic group” herein.

The term “aryl” describes an all-carbon monocyclic or fused-ringpolycyclic (i.e., rings which share adjacent pairs of carbon atoms)groups having a completely conjugated pi-electron system. The aryl groupmay be substituted or unsubstituted. Substituted aryl may have one ormore substituents, whereby each substituent group can independently be,for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl,aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide,phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy,thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate,O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea,N-carbamate, O-carbamate, C-amide, N-amide, guanyl, guanidine andhydrazine. The aryl group can be an end group, as this term is definedhereinabove, wherein it is attached to a single adjacent atom, or alinking group, as this term is defined hereinabove, connecting two ormore moieties at two or more positions thereof.

The term “heteroaryl” describes a monocyclic or fused ring (i.e., ringswhich share an adjacent pair of atoms) group having in the ring(s) oneor more atoms, such as, for example, nitrogen, oxygen and sulfur and, inaddition, having a completely conjugated pi-electron system. Examples,without limitation, of heteroaryl groups include pyrrole, furan,thiophene, imidazole, oxazole, thiazole, pyrazole, pyridine, pyrimidine,quinoline, isoquinoline and purine. The heteroaryl group may besubstituted or unsubstituted. Substituted heteroaryl may have one ormore substituents, whereby each substituent group can independently be,for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl,aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide,phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy,thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate,O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea,O-carbamate, N-carbamate, C-amide, N-amide, guanyl, guanidine andhydrazine. The heteroaryl group can be an end group, as this phrase isdefined hereinabove, where it is attached to a single adjacent atom, ora linking group, as this phrase is defined hereinabove, connecting twoor more moieties at two or more positions thereof. Representativeexamples are pyridine, pyrrole, oxazole, indole, purine and the like.

The term “halide” and “halo” describes fluorine, chlorine, bromine oriodine.

The term “haloalkyl” describes an alkyl group as defined above, furthersubstituted by one or more halide.

The term “sulfate” describes a —O—S(═O)₂—OR′ end group, as this term isdefined hereinabove, or an —O—S(═O)₂—O— linking group, as these phrasesare defined hereinabove, where R′ is as defined hereinabove.

The term “thiosulfate” describes a —O—S(═S)(═O)—OR′ end group or a—O—S(═S)(═O)—O— linking group, as these phrases are defined hereinabove,where R′ is as defined hereinabove.

The term “sulfite” describes an —O—S(═O)—O—R′ end group or a —O—S(═O)—O—group linking group, as these phrases are defined hereinabove, where R′is as defined hereinabove.

The term “thiosulfite” describes a —O—S(═S)—O—R′ end group or an—O—S(═S)—O— group linking group, as these phrases are definedhereinabove, where R′ is as defined hereinabove.

The term “sulfinate” describes a —S(═O)—OR′ end group or an —S(═O)—O—group linking group, as these phrases are defined hereinabove, where R′is as defined hereinabove.

The term “sulfoxide” or “sulfinyl” describes a —S(═O)R′ end group or an—S(═O)— linking group, as these phrases are defined hereinabove, whereR′ is as defined hereinabove.

The term “sulfonate” describes a —S(═O)₂—R′ end group or an —S(═O)₂—linking group, as these phrases are defined hereinabove, where R′ is asdefined herein.

The term “S-sulfonamide” describes a —S(═O)₂—NR′R″ end group or a—S(═O)₂—NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “N-sulfonamide” describes an R′S(═O)₂—NR″— end group or a—S(═O)₂—NR′— linking group, as these phrases are defined hereinabove,where R′ and

R″ are as defined herein.

The term “disulfide” refers to a —S—SR′ end group or a —S—S— linkinggroup, as these phrases are defined hereinabove, where R′ is as definedherein.

The term “phosphonate” describes a —P(═O)(OR′)(OR″) end group or a—P(═O)(OR′)(O)— linking group, as these phrases are defined hereinabove,with R′ and

R″ as defined herein.

The term “thiophosphonate” describes a —P(═S)(OR′)(OR″) end group or a—P(═S)(OR′)(O)— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “phosphinyl” describes a —PR′R″ end group or a —PR′— linkinggroup, as these phrases are defined hereinabove, with R′ and R″ asdefined hereinabove.

The term “phosphine oxide” describes a —P(═O)(R′)(R″) end group or a—P(═O)(R′)— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “phosphine sulfide” describes a —P(═S)(R′)(R″) end group or a—P(═S)(R′)— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “phosphite” describes an —O—PR′(═O)(OR″) end group or an—O—PH(═O)(O)— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “carbonyl” or “carbonate” as used herein, describes a —C(═O)—R′end group or a —C(═O)— linking group, as these phrases are definedhereinabove, with R′ as defined herein.

The term “thiocarbonyl” as used herein, describes a —C(═S)—R′ end groupor a —C(═S)— linking group, as these phrases are defined hereinabove,with R′ as defined herein.

The term “oxo” as used herein, describes a (═O) group, wherein an oxygenatom is linked by a double bond to the atom (e.g., carbon atom) at theindicated position.

The term “thiooxo” as used herein, describes a (═S) group, wherein asulfur atom is linked by a double bond to the atom (e.g., carbon atom)at the indicated position.

The term “oxime” describes a ═N—OH end group or a ═N—O— linking group,as these phrases are defined hereinabove.

The term “hydroxyl” describes a —OH group.

The term “alkoxy” describes both an —O-alkyl and an —O-cycloalkyl group,as defined herein.

The term “aryloxy” describes both an —O-aryl and an —O-heteroaryl group,as defined herein.

The term “thiohydroxy” describes a —SH group. The term “thioalkoxy”describes both a —S-alkyl group, and a —S-cycloalkyl group, as definedherein.

The term “thioaryloxy” describes both a —S-aryl and a —S-heteroarylgroup, as defined herein.

The “hydroxyalkyl” is also referred to herein as “alcohol”, anddescribes an alkyl, as defined herein, substituted by a hydroxy group.

The term “cyano” describes a —C≡N group.

The term “isocyanate” describes an —N═C═O group. The term“isothiocyanate” describes an —N═C═S group. The term “nitro” describesan —NO₂ group. The term “acyl halide” describes a —(C═O)R″″ groupwherein R″″ is halide, as defined hereinabove.

The term “azo” or “diazo” describes an —N═NR′ end group or an —N═N—linking group, as these phrases are defined hereinabove, with R′ asdefined hereinabove.

The term “peroxo” describes an —O—OR′ end group or an —O—O— linkinggroup, as these phrases are defined hereinabove, with R′ as definedhereinabove.

The term “carboxylate” as used herein encompasses C-carboxylate andO-carboxylate.

The term “C-carboxylate” describes a —C(═O)—OR′ end group or a —C(═O)—O—linking group, as these phrases are defined hereinabove, where R′ is asdefined herein.

The term “O-carboxylate” describes a —OC(═O)R′ end group or a —OC(═O)—linking group, as these phrases are defined hereinabove, where R′ is asdefined herein.

A carboxylate can be linear or cyclic. When cyclic, R′ and the carbonatom are linked together to form a ring, in C-carboxylate, and thisgroup is also referred to as lactone. Alternatively, R′ and O are linkedtogether to form a ring in O-carboxylate. Cyclic carboxylates canfunction as a linking group, for example, when an atom in the formedring is linked to another group.

The term “thiocarboxylate” as used herein encompasses C-thiocarboxylateand O-thiocarboxylate.

The term “C-thiocarboxylate” describes a —C(═S)—OR′ end group or a—C(═S)—O— linking group, as these phrases are defined hereinabove, whereR′ is as defined herein.

The term “O-thiocarboxylate” describes a —OC(═S)R′ end group or a—OC(═S)— linking group, as these phrases are defined hereinabove, whereR′ is as defined herein.

A thiocarboxylate can be linear or cyclic. When cyclic, R′ and thecarbon atom are linked together to form a ring, in C-thiocarboxylate,and this group is also referred to as thiolactone. Alternatively, R′ andO are linked together to form a ring in O-thiocarboxylate. Cyclicthiocarboxylates can function as a linking group, for example, when anatom in the formed ring is linked to another group.

The term “carbamate” as used herein encompasses N-carbamate andO-carbamate.

The term “N-carbamate” describes an R″OC(═O)—NR′— end group or a—OC(═O)—NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “O-carbamate” describes an —OC(═O)—NR′R″ end group or an—OC(═O)—NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

A carbamate can be linear or cyclic. When cyclic, R′ and the carbon atomare linked together to form a ring, in O-carbamate. Alternatively, R′and O are linked together to form a ring in N-carbamate. Cycliccarbamates can function as a linking group, for example, when an atom inthe formed ring is linked to another group.

The term “carbamate” as used herein encompasses N-carbamate andO-carbamate.

The term “thiocarbamate” as used herein encompasses N-thiocarbamate andO-thiocarbamate.

The term “O-thiocarbamate” describes a —OC(═S)—NR′R″ end group or a—OC(═S)—NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “N-thiocarbamate” describes an R″OC(═S)NR′— end group or a—OC(═S)NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

Thiocarbamates can be linear or cyclic, as described herein forcarbamates.

The term “dithiocarbamate” as used herein encompasses S-dithiocarbamateand N-dithiocarbamate.

The term “S-dithiocarbamate” describes a —SC(═S)—NR′R″ end group or a—SC(═S)NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “N-dithiocarbamate” describes an R″SC(═S)NR′— end group or a—SC(═S)NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “urea”, which is also referred to herein as “ureido”, describesa —NR′C(═O)—NR″R′″ end group or a —NR′C(═O)—NR″— linking group, as thesephrases are defined hereinabove, where R′ and R″ are as defined hereinand R′″ is as defined herein for R′ and R″.

The term “thiourea”, which is also referred to herein as “thioureido”,describes a —NR′—C(═S)—NR″R′″ end group or a —NR′—C(═S)—NR″— linkinggroup, with R′, R″ and R′″ as defined herein.

The term “amide” as used herein encompasses C-amide and N-amide.

The term “C-amide” describes a —C(═O)—NR′R″ end group or a —C(═O)—NR′—linking group, as these phrases are defined hereinabove, where R′ and R″are as defined herein.

The term “N-amide” describes a R′C(═O)—NR″— end group or a R′C(═O)—N—linking group, as these phrases are defined hereinabove, where R′ and R″are as defined herein.

An amide can be linear or cyclic. When cyclic, R′ and the carbon atomare linked together to form a ring, in C-amide, and this group is alsoreferred to as lactam. Cyclic amides can function as a linking group,for example, when an atom in the formed ring is linked to another group.

The term “guanyl” describes a R′R″NC(═N)— end group or a —R′NC(═N)—linking group, as these phrases are defined hereinabove, where R′ and R″are as defined herein.

The term “guanidine” describes a —R′NC(═N)—NR″R′″ end group or a—R′NC(═N)—NR″— linking group, as these phrases are defined hereinabove,where R′, R″ and R′″ are as defined herein.

The term “hydrazine” describes a —NR′—NR″R′″ end group or a —NR′—NR″—linking group, as these phrases are defined hereinabove, with R′, R″,and R′″ as defined herein.

As used herein, the term “hydrazide” describes a —C(═O)—NR′—NR″R′″ endgroup or a —C(═O)—NR′—NR″— linking group, as these phrases are definedhereinabove, where R′, R″ and R′″ are as defined herein.

As used herein, the term “thiohydrazide” describes a —C(═S)—NR′—NR″R′″end group or a —C(═S)—NR′—NR″— linking group, as these phrases aredefined hereinabove, where R′, R″ and R′″ are as defined herein.

As used herein, the term “alkylene glycol” describes a—O—[(CR′R″)_(z)—O]_(y)—R′″ end group or a —O—[(CR′R″)_(z)—O]_(y)—linking group, with R′, R″ and R′″ being as defined herein, and with zbeing an integer of from 1 to 10, preferably, from 2 to 6, morepreferably 2 or 3, and y being an integer of 1 or more. Preferably R′and R″ are both hydrogen. When z is 2 and y is 1, this group is ethyleneglycol. When z is 3 and y is 1, this group is propylene glycol. When yis 2-4, the alkylene glycol is referred to herein as oligo(alkyleneglycol).

When y is greater than 4, the alkylene glycol is referred to herein aspoly(alkylene glycol). In some embodiments of the present invention, apoly(alkylene glycol) group or moiety can have from 10 to 200 repeatingalkylene glycol units, such that z is 10 to 200, preferably 10-100, morepreferably 10-50.

The term “silanol” describes a —Si(OH)R′R″ group, or —Si(OH)₂R′ group or—Si(OH)₃ group, with R′ and R″ as described herein.

The term “silyl” describes a —SiR′R″R′″ group, with R′, R″ and R′″ asdescribed herein.

As used herein, the term “urethane” or “urethane moiety” or “urethanegroup” describes a Rx-O—C(═O)—NR′R″ end group or a —Rx-O—C(═O)—NR′—linking group, with R′ and R″ being as defined herein, and Rx being analkyl, cycloalkyl, aryl, alkylene glycol or any combination thereof.Preferably R′ and R″ are both hydrogen.

The term “polyurethane” or “oligourethane” describes a moiety thatcomprises at least one urethane group as described herein in therepeating backbone units thereof, or at least one urethane bond,—O—C(═O)—NR′—, in the repeating backbone units thereof.

It is expected that during the life of a patent maturing from thisapplication many relevant curable materials featuring properties (e.g.,Tg when hardened) as described herein, will be developed, and the scopeof the respective curable materials is intended to include all such newmaterials a priori.

Various embodiments and aspects of the present invention as delineatedhereinabove and as claimed in the claims section below find experimentalsupport in the following examples.

EXAMPLES

Reference is now made to the following examples, which together with theabove descriptions illustrate some embodiments of the invention in a nonlimiting fashion.

Materials and Methods

3D Inkjet printing of shelled objects is performed using Objet C2, C3Systems, in a DM mode (e.g., a DM mode referred to as DM 5160 or 5130),according to the method described in U.S. Patent Application havingPublication No. 2013/0040091. Generally, all printed objects arecomprised of a core made of the first formulation (RF, Part A) and thesecond formulation (DLM, Part B), at a ratio as indicated, and twoshells, wherein one shell comprises the first formulation, and anothershell comprises the second formulation, as described herein for alayered structure.

HDT measurements are performed on Ceast vicat/HDT instrument accordingto ASTM D-648-06.

Print deformations (curling) are quantitatively assessed using a230×10×10 mm printed bar. Upon printing, the bar is left within theprinting system, in a closed cabinet, for 1 hour, and is thereafterstored at room temperature for 24 hours. The bar is then placed on aflat plane (flat table) and curling is measured by putting weight on oneside of the bar and measuring the height of the bar edge from the planein mm. For this study an elevation of 4 mm or less is considered asacceptable for most mainstream applications.

Tray temperature is measured directly by using Thermocouple connected todata logger apparatus.

Measurements of other properties were performed according to standardprotocols, unless otherwise indicated.

All reagents and materials composing the modeling material formulationsare obtained from known vendors.

Example 1 Exemplary Modeling Material Formulations

The following formulations present exemplary formulations successfullypracticed in AM of layered objects featuring a layered structure asdescribed herein in any of the respective embodiments.

A First Formulation:

Table 1 below presents exemplary materials suitable for inclusion in thefirst (Part A, RF) formulation, according to some embodiments of thepresent invention:

TABLE 1 Percent- Component Exemplary Materials age (%) Curable acrylicACMO (CAS: 5117-12-4) 10-40 monomer, (Tg = 88° C.) characterized, IBOA(CAS: 5888-33-5) when hardened, (Tg = 95° C.) by Tg > 85° C. SR 833S(CAS: 42594-17-2) (Tg = 185° C.) SR454 ethoxylated (3) TMPTA (CAS:28961-43-5) (Tg = 103° C.) SR508 (CAS 57472-68-1) (Tg = 104° C.) SR368(CAS: 40220-08-4) (Tg = 272° C.) Curable SR 834 (CAS: 43048-08-4) 35-50Methacrylic SR-423D (CAS: 7534-94-3) monomer, characterized, whenhardened, by Tg > 150° C. (Meth)acrylic BR-441 (Di functional Aliphatic10-40 Oligomer, polyester urethane Acrylate) characterized, (Tg = 71°C.) when hardened, PH 6019 (Trifunctional Aliphatic by Tg > 50° C.urethane acrylate) (Tg = 51° C.) Eb3703 amine modified epoxy Diacrylate(Tg = 57° C.) (Meth)acrylic SR 9036 (Ethoxylated (30) bisphenol A  5-30flexible monomer, dimethacrylate) (CAS: 41637-38-1) Having low (Tg =−43° C.) Tg < −20° C SR415 (Ethoxylated (20) Trimethylol propanetriacrylate) (CAS: 28961-43-5) (Tg = −40° C.) SR 9035 (Ethoxylated (15)Trimethylol propane triacrylate (Tg = −30° C.) SR610 (Poly(ethyleneglycol) (600) diacrylate) (Tg = −40° C.) Photoinitiator BAPO type (BisAcyl Phosphine Oxide) 0.5-5  Alpha Hydroxy ketone MAPO(Monoacylphosphine oxides) Surfactant BYK Type (PDMS derivatives)0.01-1   Dispersing agent BYK Type (PDMS derivatives) 0.01-1   InhibitorMEHQ 0.1-1  Genorad Type Inorganic Pigment Nano scale Titanium Oxide0.1-0.3 Nano scale Zirconium Oxide Nano Silica Organic pigment Nanoscale Carbon black  0.1-0.15

Tables 2 and 3 below present chemical compositions of exemplaryformulations suitable for use as the first formulation (RF), alsoreferred to as RGD531 (Table 2), and GR-71-black2 or RF71 (Table 3A).Table 3B below presents a chemical composition of an exemplary RFformulation which comprises 90% by weight of a formulation referred toherein as RF71, to which 10% by weight of SR-610, as an exemplaryadditional flexible monomer, was added, and which is referred to hereinas RF71*.

TABLE 2 Material Wt. percentage (%) Mono functional Acrylic monomer10-30 Tg > 85° C. Methacrylic monomer, Tg > 150° C.  5-20 Polyesterbased Urethane Acrylate  5-20 (Meth)acrylic Oilgomer, Tg > 50° C. 10-20Acrylic multi functional monomer Tg > 85° C. 20-30 Inhibitor 0.1-0.3Photoinitiator 0.5-3  Surfactant 0.01-1   Epoxy Acrylate 1-5 Dispersingagent 0.01-0.05 Inorganic pigment nanoscale 0.2-0.6

TABLE 3A Material Wt. percentage (%) Mono functional Acrylic monomerTg > 85° C. 10-30 Multi functional Acrylic monomer Tg > 85° C. 10-20 SR834 ≥35% (Meth)acrylic Oligomer, Tg > 50° C. 10-20 (Meth)acrylicflexible monomer, Having low  5-15 Tg < −20° C. Inhibitor 0.1-0.3Photoinitiator 2-5 Surfactant 0.01-1   Dispersant 0.01-1   Organicpigment  0.5-0.15

TABLE 3B Material Wt. percentage (%) Mono functional Acrylic monomerTg > 85° C. 9-27 Multi functional Acrylic monomer Tg > 85° C. 9-18 SR834 ≥32% (Meth)acrylic Oligomer, Tg > 50° C. 9-18 (Meth)acrylic flexiblemonomer, Having low 4.5-13.5 Tg < −20° C. Inhibitor 0.09-0.3 Photoinitiator 1.8-4.5  Surfactant 0.01-1    Dispersant 0.01-1   Organic pigment 0.5-0.15 An additional (Meth)acrylic flexible monomer,5-15 Having low Tg < −20° C.

Table 4 below presents the chemical composition of an additionalexemplary Part A formulation (RF4w).

TABLE 4 Material Wt. percentage (%) Mono functional Acrylic monomer Tg >85° C. 10-30 Multi functional Acrylic monomer Tg > 85° C. 10-30 SR834<35% Polyester Urethane Acrylate  5-15 (Meth)acrylic flexible monomer,Having low 10-30 Tg < −20° C. Inhibitor 0.1-0.3 Photoinitiator 1-5Surfactant 0.01-1   Dispersant 0.01-1   Inorganic pigment 0.5-1 

The Second Formulation:

Table 5 below presents exemplary materials suitable for inclusion in thesecond (Part B, DLM) formulation, according to some embodiments of thepresent invention:

TABLE 5 Material Examples Percentage (%) Curable (meth)acrylic ACMO(CAS: 5117-12-4) (Tg = 88° C.)  10-50 monomer, characterized, IBOA(CAS:5888-33-5) (Tg = 95° C.) when hardened, by SR 833S (CAS: 42594-17-2) (Tg= 185° C.) Tg > 85° C. SR454 ethoxylated (3) TMPTA (CAS 28961-43-5) (Tg= 103° C.) SR508 (CAS 57472-68-1) (Tg = 104° C.) SR368 (CAS 40220-08-4)(Tg = 272° C.) SR423 (CAS 7534-94-3) (Tg = 110° C.) Curable(meth)acrylic CN-991 (Aliphatic polyester based  10-50 oligomercharacterized, Urethane diacrylate) (Tg = 40° C.) when hardened, by PH6019 Aliphatic Urethane TriAcrylate Tg > 10° C. (Tg = 51° C.) Eb3708(Modified bisphenol-A epoxy diacrylate) (Tg = 21° C.) Curableethoxylated SR 9036 (Ethoxylated (30) bisphenol A   5-40 trifunctional(meth)acrylic dimethacrylate) (CAS 41637-38-1) (Tg = −43° C.) monomer,characterized, SR415 (Ethoxylated (20) Trimethylol propane whenhardened, by triacrylate) (CAS 28961-43-5, Tg = −40° C.) Tg < −20° C. SR9035 (Ethoxylated (15) Trimethylol propane triacrylate) (Tg = −30° C.)SR610 (Poly (ethylene glycol) (600) diacrylate) (Tg = −40° C.) *Otherexamples are shown in Table 6 below Photoinitiator BAPO type (Bis AcylPhosphine Oxide) 0.5-5 Alpha Hydroxy ketone MAPO (Monoacylphosphineoxides) Surfactant BYK Type (PDMS derivatives) 0.01-1  Dispersing agentBYK Type (PDMS derivatives) 0.01-1  Inhibitor MEHQ Genorad Type 0.1-1

Table 6 below presents exemplary ethoxylated materials, and theirproperties, which are suitable for inclusion in the second formulation(Part B).

TABLE 6 Number of Viscosity MW Ethoxylated (Cp at 25° Material(gram/mol) groups C.) SR- Ethoxylated (30) 2156 30 610 9036 bisphenol Adimethacrylate SR-415 Ethoxylated (20) 1176 20 225 TrimethylolpropaneTriacrylate SR430 Ethoxylated 18 1249 18 825 Tristyrylphenol acrylate(RSP(18EO)A) SR9035 Ethoxylated 15 956 15 177 TrimethylolpropaneTriacrylate SR567P Ethoxylated 25 C22 1494 25 250 methacrylate SR480Ethoxylated 10 808 10 410 bisphenol A DMA SR499 Ethoxylated (6) 554 6 92Trimethylolpropane Triacrylate SR610 Poly(ethylene glycol) 726 13 100(600) diacrylateElastic Moduli Ratio:

In some embodiments of the present invention, the first and the secondmodeling formulations are selected according to their characteristicelastic moduli. Computer simulations have been conducted in order todetermine a preferred ratio between the elastic moduli of the twomodeling formulations. The computer simulations were performed forvarious combinations in which the first modeling formulation is aformulation that is commercially available under the trade name RGD531,and having an elastic modulus of 3000 MPa. Seven types of formulationswere tested as the second formulations. These are referred to asSoft-30, Soft-16, RGD515, M-1, M-2, M-3, and M-4.

The computer simulations included analysis of stress distributionresulting from a crack in the second modeling formulation. The resultsof the simulations are provided in Table 7 and FIGS. 1A-G. In FIGS.1A-G, the lower layer corresponds to the first modeling formulation(RGD531, in the present Example), and the upper layer corresponds to therespective second modeling formulation (Soft-30, Soft-16, RGD515, M-1,M-2, M-3, and M-4, respectively)

TABLE 7 Second Young's modulus Formulation [MPa] Max stress value andlocation Soft-30 90 400 MPa in the first modeling formulation Soft-16550 283 MPa in the first modeling formulation RGD515 1000 250 MPa in thefirst modeling formulation 250 MPa at the interface between the twoformulations, under the crack M-1 1330 main stress of 250 MPa at thebottom of the crack and at the interface between the two formulations,under the crack 200 MPa in the first modeling formulation M-2 1600 mainstress of 257 MPa at the bottom of the crack and at the interfacebetween the two formulations under the crack 257 MPa in the firstmodeling formulation M-3 1700 main stress of 269 MPa at the bottom ofthe crack 220 MPa at the interface between the two formulations, underthe crack M-4 1800 main stress of 285 MPa at the bottom of the 220 MPaat the interface between the two formulations, under the crack

Table 7 demonstrates that there is a ratio between the elastic modulifor which the distribution of stress is optimal. In the present example,the optimal distribution of stress is achieved when the elastic modulusof the second modeling formulation is from about 1000 to about 1330,corresponding to a ratio between the elastic moduli of from 2.7 to 3.0.

Effect of RF Concentration in the Core:

Further the effect of the concentration of the first formulation (PartA, RF) in the core region on the HDT of the core and of the final objectwas tested.

In exemplary measurements, a Part A formulation referred to herein asRF71 was used in combination with a Part B formulation, referred toherein as DI-69.

Samples having a thickness of 6.35 mm were printed as follows:

DM-ABS: Full DM 5160 Structure

DM-ABS PC: same with thermal post curing

RND: Only the Core structure, Random DM

RND PC: Same with thermal post curing

FIG. 2 presents the effect of various concentrations of RF71 in the coreon the HDT of the various printed objects, 6.35 mm in thickness.

As can be seen in FIG. 2, increasing the amount of the Part Aformulation in the core region increases the HDT of the core. However,in the ABS DM mode for obtaining a layered structure as describedherein, when the core is surrounded by inner and outer envelope regionsas described herein, the overall HDT is generally the same,irrespectively of the percentage of the first modeling formulation inthe core, indicating that the HDT of the sample is not affected by therelative amount of Part A in the core.

Example 2 Controlling Viscoelastic Properties of the Object

Three types for the first modeling formulation, and several ratiosbetween the amounts (weight, in the present example) of the first andsecond modeling formulations were experimented in the present Example,for the purpose of controlling the viscoelastic properties of theobject. The experiments included fabrication of a rectangle of 3 mmoverall thickness, overall length of 30 mm, with 17 mm between the twofixtures (span), and 13 mm width, by a three-dimensional inkjet printingsystem. The thickness of each layer was 32 μm. Each layer was printed byrandom interlacing of the respective first modeling formulation and asecond modeling formulation to form a digital material. The respectivefirst modeling formulation was selected to provide, when hardened, acured material which is more rigid than the cured material provided bythe second modeling formulation, when hardened, and wherein therespective first modeling formulation has a component with a higher Tgthan any of the components of the second modeling formulation. By“component” it is meant either a non-curable polymeric material or acurable material that forms, when hardened, a polymeric materialfeaturing the indicated Tg.

The three types of the first modeling formulation used in theexperiments are denoted herein as RF 4w, RF 535 and RF 71. The totalcalculated Tg values for these three types of first modeling formulation(obtained by sum of the individual Tg values of the respectivecomponents, weighted by the respective weight percentage, of eachmodeling formulation) are 127° C., 107° C. and 146° C.

Formulation RF 4w included a mono-functional oligomeric curablemethacrylic material, and a mono-functional oligomeric curable acrylicmaterial, each featuring a relatively high Tg when hardened (such as,but not limited to, SR-834 and SR-833), and a di-functional oligomericcurable acrylic material featuring a low Tg, such as, but not limitedto, SR-9036 (see Example 1), functioning, inter alia, as a cross-linkingagent, with a ratio of about 2.4 between the total amount of themono-functional curable materials and multi-functional curable material.Formulation RF71 included the same combination of mono-functional anddi-functional curable materials featuring high Tg, with a ratio of about5.5 between the total amount of the mono-functional curable materialsand the multi-functional curable material. Formulation RF 535 was devoidof the methacrylic monofunctional curable material and of thedi-functional curable material, and included the monofunctional acryliccurable material as in the other formulations in combination withanother monofunctional methacrylic curable material featuring a lower Tg(e.g., IBOMA; CAS No. 7534-94-3). Thus RF 535 is less cross-linkedcompared to RF 4w and RF 71.

Dynamic mechanical analysis was performed for the fabricated layeredcores. The dynamic mechanical analysis provided the trigonometricfunction tan(δ), where δ is the phase between the stress σ and thestrain ε. The function tan(δ) is a viscoelastic property that iscorrelated with the damping of the respective formulation, or, morespecifically, the ability of the respective formulation to dissipatemechanical energy by converting the mechanical energy into heat.Measurements were performed using a digital mechanical analysis (DMA)system, model Q800, available from TA Instruments, Inc., of New Castle,Del. The DMA system was operated in a single cantilever mode,oscillation mode, temperature ramp, frequency of 1 Hz and heating rateof 3° C./min.

The results are shown in FIGS. 13A-E.

FIGS. 13A and 13B show tan(δ) as a function of the temperature, for 25,50, 70 and 90 weight percentage of the first modeling formulation in thecore. The first modeling formulation in FIG. 13A is RF 4w, and the firstmodeling formulation in FIG. 13B is RF 535. FIG. 13C is the same asFIGS. 13A and 13B, but for RF 71, and for 25, 50 and 90 weightpercentage of the first modeling formulation in the layered core. Asshown in FIGS. 13A, 13B and 13C, for all three types of the firstmodeling formulation, the location and width of the peak of tan(δ) as afunction of the temperature vary smoothly and monotonically when theweight percentage of the first modeling formulation is increased.

FIG. 13D shows tan(δ) as a function of the temperature for the threetypes of the first modeling formulation RF 4w, RF 535 and RF 71, at a25/75 weight percentage ratio between the first and second modelingformulation. FIG. 13E is the same as FIG. 13D, but for a 50/50 weightpercentage ratio. As shown in FIGS. 13D and 13E, formulation RF 535exhibits a higher peak at lower temperature, both for the 25/75 ratioand for the 50/50.

This example demonstrates that one or more parameters characterizing thefirst formulation, and/or the relative amount of the first and secondformulations can be selected to enhance or reduce the damping of thefabricated object. For example, selection of the first formulationand/or ratio that enhance the damping provides an object that is capableof dissipating energy, and that is less sensitive to internal stressesand crack propagation. The selected parameter characterizing the firstformulation can be the extent of cross linking of the first formulation(e.g., the relative amount of a multi-functional curable material), thetotal calculated Tg of the first formulation as calculated by summingthe individual Tg values of polymeric components and the like.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting.

What is claimed is:
 1. A method of layerwise fabrication of athree-dimensional object, the method comprising, for each of at least afew of the layers: dispensing at least a first modeling formulation anda second modeling formulation to form a core region using both saidfirst and said second modeling formulations, an inner envelope region atleast partially surrounding said core region using said first modelingformulation but not said second modeling formulation, and an outerenvelope region at least partially surrounding said inner enveloperegion using said second modeling formulations but not said firstmodeling formulation; exposing said layer to curing energy, therebyfabricating the object, wherein each of said first modeling formulationand said second modeling formulation comprises at least one UV-curablematerial, and wherein said first modeling formulation and said secondmodeling formulation differ from one another, when hardened, by at leastone of: Heat Deflection Temperature (HDT), Izod Impact resistance, Tgand elastic modulus.
 2. The method according to claim 1, wherein an HDTof said first modeling material formulation, when hardened, is higherthan an HDT of said second modeling material formulation, when hardened.3. The method according to claim 1, wherein an HDT of said secondmodeling material formulation, when hardened, is lower than 50° C. andan HDT of said first modeling material formulation, when hardened, ishigher than 50° C.
 4. The method according to claim 1, wherein an IzoDImpact Resistance of said second modeling material formulation, whenhardened, is higher than an Izod Impact Resistance of said firstmodeling material formulation, when hardened.
 5. The method according toclaim 1, wherein a ratio between elastic moduli of said first modelingmaterial formulation and said second modeling formulation, whenhardened, ranges from 1 to 20, or from 1 to 10, or from 1 to 5, or from2 to 5, or from 2 to 3, or from 2.5 to 3, or from 2.7 to 2.9.
 6. Themethod according to claim 1, wherein said first modeling materialformulation comprises at least one curable material that ischaracterized, when hardened, by Tg of at least 50° C.
 7. The methodaccording to claim 6, wherein said first modeling material formulationcomprises at least two curable materials, at least one of said curablematerials is characterized, when hardened, by Tg of at least 80° C. 8.The method according to claim 1, wherein said first modeling materialformulation comprises at least two curable materials, at least one ofsaid curable materials is characterized, when hardened, by Tg of atleast 100° C., or at least 150° C.
 9. The method according to claim 6,wherein said first modeling material formulation comprises: at least onecurable (meth)acrylic monomer; at least one curable (meth)acrylicoligomer; and optionally, at least one curable (meth)acrylic monomercharacterized, when hardened, by Tg lower than 0° C.
 10. The methodaccording to claim 6, wherein said first modeling material formulationcomprises: at least one curable (meth)acrylic monomer characterized,when hardened, by Tg of at least 85° C.; at least one curable(meth)acrylic monomer characterized, when hardened, by Tg of at least150° C.; at least one curable (meth)acrylic oligomer, characterized,when hardened, by Tg of at least 50° C.; and optionally, at least onecurable (meth)acrylic monomer characterized, when hardened, by Tg lowerthan 0° C.
 11. The method according to claim 1, wherein said secondmodeling material formulation comprises at least two curable materials,at least one of said curable materials is a (meth)acrylic monomercharacterized, when hardened, by Tg lower than −20° C.
 12. The methodaccording to claim 1, wherein a thickness of said inner envelope region,as measured within a plane of said layer and perpendicularly to asurface of the object, is preferably from about 0.1 mm to about 4 mm.13. The method according to claim 1, wherein a thickness of said outerenvelope region, as measured within a plane of said layer andperpendicularly to a surface of the object, is from about from about 150microns to about 600 microns.
 14. The method according to claim 1,wherein said dispensing is executed to form at least one additionalenvelope region between said inner envelope region and said outerenvelope region.
 15. The method according to claim 14, wherein saiddispensing of said additional envelope region is using both said firstand said second modeling formulations.
 16. The method according to claim14, wherein a thickness of said additional envelope, as measured withina plane of said layer and perpendicularly to a surface of the object, isless than a thickness of said inner envelope region and also less than athickness of said outer envelope region.
 17. The method according toclaim 14, wherein a thickness of said additional envelope, as measuredwithin a plane of said layer and perpendicularly to a surface of theobject, is from about 70 microns to about 100 microns.
 18. The methodaccording to claim 15, wherein a ratio between a number of voxels withinsaid additional envelope region that are occupied by said first modelingformulation and a number of voxels within said additional enveloperegion that are occupied by said second modeling formulation is about 1.19. The method according to claim 1, further comprising dispensing aplurality of base layers to form a base section of the object, saidplurality of base layers comprising at least one outer base layer madeof said second modeling formulation but not said first modelingformulation, and at least one inner base layer made of said firstmodeling formulation but not said second modeling formulation.
 20. Themethod according to claim 1, further comprising dispensing a pluralityof top layers to form a top section of the object, said plurality of toplayers comprising at least one outer top layer made of said secondmodeling formulation but not said first modeling formulation, and atleast one inner top layer made of said first modeling formulation butnot said second modeling formulation.
 21. The method according to claim1, wherein at least one parameter characterizing said first formulationis selected to provide a predetermined damping for said core.
 22. Themethod according to claim 21, wherein said at least one parametercomprises an extent of cross linking of the first modeling formulation.23. The method according to claim 22, wherein said at least oneparameter comprises a total calculated Tg of said first formulation, ascalculated by summing individual Tg values of polymeric materialsincluded in said first modeling formulation, when hardened.
 24. Themethod according to claim 1, wherein relative amounts of said first andsaid second formulations is selected to provide a predetermined dampingfor said core.